Combat Mode

You enable combat mode with 4, disable it with 1 or toggle it with F. You can also click the icon to the bottom right. Replaced intents in 2021.

Left-clicking mobs/players with an empty hand will harm them if on, or help them if off (see below for details). Also prevents you from switching places or being pushed when colliding. Should be kept off by default so you don't block people.

Some items require this mode to be off or you will wield them as improvised weapons and bash things.

Exceptions

Clicking on another character with a syringe or food in your hand will make you attempt to inject/forcefeed them regardless of your current mode. This will always produce a message about you forcefully injecting/feeding them, which your target will see this message in big red letters on their screen, so be sure to warn them first if you're actually trying to be helpful.
Items with no helpful use will still be used to bash even while in Non-Combat Mode. Remember that some items require you to click on them while they're in your hand, so try that if you just bashed yourself by accident.

Help

When NOT in combat mode, left-click people with an empty hand to help them:

If they're in crit, you'll perform CPR in an attempt to revive them. Make sure both of you have your mouths free (remove their mask by dragging their body onto yours)!
If they're on the floor and not in crit, you'll try to help them back up on their feet. Since being knocked down stops you from performing most actions, this can be a lifesaver.
Otherwise, you'll give them a hug.

Disarm

In any mode (combat mode on or off doesn't matter) right-click is used to disarm/shove.

This is used to non-lethally disarm or incapacitate people. If you right-click on someone with an empty hand, you'll shove them.

Shoving someone pushes them one tile away from you. If there is nothing blocking them, they are slowed down very slightly for 3 seconds. If you shove someone again while they are slowed, it knocks ranged weapons out of their active hand.

If the blocking tile has a table on it, they're pushed onto it knocking them over for 3 seconds.
If the blocking tile has another human on it, it knocks them both over, the collateral victim for 1.1 second instead of 3.
If the tile is blocked by something else, it knocks them down for 3 seconds.
If the target is already knocked down (from any source, including slipping), shoving them will paralyze for 3 seconds. This paralyze cannot be chained or extended, so shoving them again while they're paralyzed does not extend the length.
You can not shove someone while standing on their tile.

It's also important to note that these actions only work if you're right-clicking on the target with an empty hand.

Grab

In any mode (combat mode on or off doesn't matter), ctrl+click people or objects to grab.

If used on an object/item you will start pulling it. Press H on your keyboard or click "pull" in the bottom right to release. If used on humanoid people you'll do a passive (lvl 1) grab, which makes you pull them. Dragging prone people will slow you down. While on passive grab, control-click the person once more to upgrade it to an aggressive grab (lvl 2). Repeat again for a neck grab (lvl 3), and again for a strangling grab (lvl 4). The target can press "resist" to try to escape from an aggressive or above grab level. If you do not have Боевой режим Включается при помощи клавиши 4, выключается при помощи клавиши 1 или может быть переключен при помощи клавиши F при стандартной раскладке. Нажатие левой кнопки на людей пустой рукой с включённым боевым режимом приведёт к удару или приведёт к помощи если он отключен. Также позволяет вам не меняться местами с человеком или толкнуть вас если вы столкнулись. on while dragging someone, you will release if you control-click empty floor.

Once you have an aggressive grab, you can do the following:

Throw the person (by enabling throwing, then clicking somewhere).
Fireman carry them if they're prone (by click dragging their sprite onto yours).
Control-click them again to slowly upgrade the grab to a level 3 neck grab.

With neck grab you deal no damage, but it's harder for them to resist and get away.

Control-click them one last time to slowly upgrade the grab to level 4, strangling.

When strangling you deal suffocation damage to the target, and the target has the lowest chance (20% per try) to successfully resist out of the grab. Failing to resist out of a grab will deal some stamina damage to you.

Harm

When in combat mode, left-click with an empty hand to harm.

On standing targets you will attempt to punch them, which deals 1.5-15 stamina and 1-10 brute damage. The chance to miss a punch increases the more stamina and brute damage you have. Punching has a chance to knockdown the target for at least 4 seconds. The more brute and stamina damage the target has, the longer the target will be knocked down. If used on prone targets you will kick them, which deals 1.5-15 brute damage, with no chance to miss.

For more controls see Keyboard Shortcuts.