Quickened, slowed, and stunned are the primary ways you can gain or lose actions. The rules for how this works appear here. All these conditions alter how many actions you regain at the start of your turn. Gaining quickened or slowed on your turn doesn’t adjust your actions that turn. If you get stunned on your turn, first complete any action or activity you’re in the middle of. If the stunned condition has a value, lose remaining actions to reduce your stunned value rather than waiting until your next turn.

Some effects are even more restrictive. Certain abilities, instead of or in addition to changing the number of actions you can use, say specifically that you can't use reactions. The most restrictive form of reducing actions is when an effect states that you can't act: this means you can't use any actions, or even speak. When you can't act, you still regain your actions unless another effect (like the stunned condition) prevents it.