Learn Finnish: Jos hyökkääjä yllättää sinut, joudut toimimaan nopeasti. - If an attacker surprises you, you will end up having to act quickly.

This is because of the verb joutua (to end up having to). It is a "phase verb" that strictly requires the next verb to be in the 3rd infinitive illative form, which ends in -maan or -mään. If the sentence used alkaa (to begin), it would take the basic 1st infinitive (alat toimia), but joutua always takes the -maan/-mään ending.

Does joudut toimimaan mean the exact same thing as sinun täytyy toimia?

They both mean you have to act, but joutua adds a specific nuance. While täytyy or pitää is a neutral "must", joutua implies ending up in a situation where you are forced to do something due to circumstances outside your control, often something negative or involuntary.

Why is the object "you" written as sinut rather than the partitive sinua?

The verb yllättää (to surprise) takes a total object here because the action is seen as complete—the attacker successfully surprises you. For personal pronouns like sinä, the total object takes a special accusative form ending in -t, which gives us sinut.

How does the noun hyökkääjä get its meaning?

It is built directly from the verb hyökätä (to attack). When you take the strong stem of the verb (hyökkää-) and add the suffix -jä, it creates a noun meaning the person doing that action. It works exactly like adding "-er" to a verb in English to get "attacker".