When do I fight?

You fight when you can win a fight and take an objective (Tower, Barracks, Roshan) because winning a fight and not claiming anything afterwards makes the fight mean almost nothing.

If you come from a different game like CoD or anything deathmatch based you will automatically assume that kills mean advantage. The more you kill the enemy heroes the stronger you become and stuff because of killstreak rewards and whatsoever. But that's not the case in DotA. When you kill an enemy hero, you recieve gold and XP based on that hero's current killstreak and XP difference from you. This means that whenever you get a kill you raise a bounty on yourself. Indeed you get the gold for the kill and the XP, but the game also puts more reward on your own head. What I am trying to say is that any advantage you gain from kills is never truly yours. Once you make a mistake all of it will be regained by the enemy team. In the other hand when you destroy a tower or barracks (maybe kill Roshan) the gold and map control you gain is actually yours and it can never be reclaimed by the enemy team. However there is a really important connection between kills and real objectives: When you get kills you will be able to easily claim objectives with it, gaining a real not reclaimable lead for your team.

I feel like this is one of the biggest mistakes even good players make: You are rampaging around the map, killing the enemy supports while you are not taking real objectives on the back of it. Once you make a mistake and die you will give away all of that perhaps to the enemy team's carry who just needed that 1K gold for a major item.

But to answer your question, you want to fight when you know you can win and you can take an objective afterwards. But how do you know you can win the fight?

• Major item pickup on one of your heroes. Typically it's a Blink Dagger on your initiator or a Mekansm on anyone who was farming it. Black King Bar on your carry is also good to have.
• Availability of ultimates. Zeus just hit level 11 and his ulti is on CD. They just used I don't know maybe Black Hole to score an easy kill on your offlaner. He has buyback and they are pushing your offlane T1 with Enigma. This is a good time to fight if the two teams are roughly even otherwise.
• Forcing the enemy team to fight when they don't want to. If you know that they can't afford to fight because they know they can't win, go ahead and pressure them.

Honestly there are so many factors to consider here, you have to think about them on your own. Mostly you want to be the enemy team to go "Oh crap." when you reveal your advantage and fight. When Na'Vi lost to Alliance at The International 3 it was kinda like this. Dendi was doing stupidly well as Templar Assassin, but Bulldog managed to keep up in terms of objectives. Na'Vi had the lead but it was mostly because Dendi was slaying everyone racking up huge a huge XP lead and a beyond godlike streak. Then Na'Vi tried to get Roshan and Alliance managed to force them into a fight they were sort of comfortable with. Shit went down, Na'Vi made mistakes, Dendi died and S4 took aegis and Dendi's streak. There you go game is completely equal again thanks to the fact that Na'Vi couldnt secure their lead well enough with taking more objectives than Alliance.

Let's say you're playing Anti-Mage and farming away in your lane. 10 minutes in you have Boots of Speed and Ring of Health, plus a bit of gold. Your lane is fine, but your mid and top T1 are both dead, and the push on your T2s is probably going to come soon. The score is 6-2 in the enemy's favour.

In this example you've got a hero who really wants to farm, in a period of the game where farming is almost always his priority since he wants to finish his Battle Fury. However, if you just keep farming then your team is almost certainly going to get steamrolled and the game will be all but lost.

In this situation then, you need to fight so your team has a realistic chance of getting back into the game. The most sensible choice would be to abandon your expensive Battle Fury(an item that Anti-Mage buys mainly to accelerate his farming) and instead build something cheap like Vanguard or Yasha that will let you have a bit more of an impact.

Now take the same situation, but let's imagine it's 6-2 in your favour and despite losing the early towers, you're confident that your team can hold the T2s and maybe pick up some kills without you. In that case, the best thing for your team would probably be for you to just keep farming up your Battle Fury while the enemy heroes are kept busy elsewhere, or even just to solo push the tower in your lane while the fight is going on. That will help secure your team's late game, and you're confident that the game will last long enough for it to pay off. Do still buy a TP scroll though, in case you need to join the fight or simply get a good opportunity to mop up kills.

Your item choice will generally depend on the hero, but fighting early tends to favour cheap stat or damage items since cheaper items are usually more cost efficient and can be completed earlier. Drum of Endurance, Mekansm, Vanguard, Armlet of Mordiggian, Yasha, and an early Black King Bar are typical choices if you're fighting early.

If you're not expecting to fight early, you may want to skip those items in favour of more expensive items that'll take longer to complete, but be relevant later into the game. This applies particularly to 'farm accelerators' - items that increase your farming speed once you get them. Examples are Hand of Midas, Radiance, Battle Fury, and Mjollnir.

Fighting

For this comment "fighting" will be any time one or more of your teams heroes is intentionally exchanging damage (via right clicks and abilities) with one or more of your opponents heroes.

There are two general scenarios behind when you want to fight:

When you have to fight to avoid losing the game.

• Enemy is pushing a tower/rax that if they destroy will make it next to impossible for you to comeback and win. You have weaker late game and are already behind.
• Enemy is pushing the final rax and will get megacreeps.
• Enemy is pushing for your throne.
• Enemy is going to rosh which, if successful, will allow them to easily win one of the fights described above.

When you know you can win the fight.

• One or more enemy heroes have made a mistake (bad positioning, wasting an important ultimate ability) and your team is in position to capitalize on it.
• You have the clear advantage when it comes to who should win the fight (better ultimate abilities, some meaningful item advantage like a Black King Bar or Mekansm, you outnumber them and their missing heroes can't get to the fight location quickly, huge advantage in gold/levels, you have an aegis on a powerful hero, etc).

There are also times you want to "feint a fight", meaning its not a situation above where you would clearly want to fight, but by making your opponent think you are going to fight them (sometimes only achievable by actually fighting them despite you most likely losing the fight):

• If when you feint fighting you can force them to buyback, and/or the lesser benefit of forcing them to group as 5 and stop farming.
• If when you feint fighting you can make them not achieve some objective like a tower push, rax push, throne push, or roshan. Sometimes while you don't think you can win the fight, they might not be confident they can win either, so feinting a tower defence or rosh fight, might be enough to scare them back.
• If when you feint fighting you can prevent them from defending some objective. Sometimes you can feint a fight (pretending to push, defend a push, or roshan) you can open the map up for one of your other heroes or group of heroes to split push a tower/rax or sneak a roshan. This is a huge part of properly split pushing an enemy: make them think you will fight them one place, when you have your split push hero (Nature's Prophet, Weaver, Spectre, Morphling, Broodmother, etc) push a tower on the other side of the map. You can also deploy the same technique to allow a hero who can solo rosh to sneak in and snag an easy rosh kill, despite the fact that your opponent would be able to easily win a fight at the rosh pit.

One of the keys to feinting a fight is to force your opponent to commit to the fight (force them to use tps or walk to the fight location, and they use abilities or items) but then to try and minimize the damage you receive (avoid any of your team's heroes dying, avoid using any big ultimates, avoid losing any objectives after the fight is over). In some scenarios (forcing buybacks) you can avoid any losses, by retreating immediately after they fall for the feint. In other scenarios (split pushing or roshing) you have to commit more and draw the fight out in order for your team-mates to achieve their alternative objectives.

So any time one of the above scenarios isn't present, you don't have some advantage and your opponent isn't forcing any fights you have to take. You should be looking to create one of the scenarios in 2 (when you know you can win the fight) or create the opportunity to do one of the scenarios in "feinting a fight". This can be several ways:

• Using Smoke, warding/dewarding, or abilities like Beastmaster Hawk, Io Relocate, or invis heroes to gain a positional advantage or a hero advantage, thus creating a scenario of a fight you can win.
• Roshing and getting an aegis giving you the extra life and thus creating a scenario of a fight you can win.
• And...

Farming

Farming is the simplest way of gaining an advantage over your opponent, allowing you to create scenarios where you can win fights by having better items or more levels than your opponent. There are two main scenarios in which you will want to farm:

1. You are not presented with a scenario in which you want to fight (they are not forcing a fight, you can't gain anything by feinting, and you do not have the advantage to win fights) and you can attempt to farm until a scenario in which you want to fight presents itself.
2. You have the advantage and could force fights, but are not confident you will win them handedly. You are confident that you can either out farm (gain gold and experience more quickly than your opponent) and/or have the advantage late game (have heroes who will benefit more from gold/levels than your opponents heroes can/will). Instead of forcing your opponent to fight by going for objectives, you can instead and sit back and farm and increase your advantage.

Whenever you are farming you must make sure you do not create opportunities for your opponent to achieve a "want to fight scenario 2 (when they know they can win the fight)". You'd do this if you were to farm in a bad position: a place where your team-mates can't help you (no tower to tp to, or they just aren't close enough), or a place where you won't be able to help your team-mates (you have no tp to get to your team-mates, or you aren't close enough to your team-mates to help them).

You should also note, as a hero who doesn't necessarily need gold/levels (support hero, or a hero who is already 16 and has their core items) it would never be as beneficial to farm as it would be to instead create opportunities for scenarios where you can win a fight, or prevent the opponent from having a scenario where they can win a fight. Often this is just achieved through "scouting", and being near your team-mates, ready to attack or defend as the necessary scenarios present themselves.