Learn Swedish: Ibland måste man låta bli att reagera. - Sometimes you must refrain from reacting.

Låta bli att is a set phrase meaning to refrain from or to intentionally not do something. Even though English uses an -ing form here (refrain from reacting), Swedish uses the infinitive marker att followed by the infinitive verb reagera. You can think of it literally as let be to react.

Why is the word order måste man and not man måste?

This is a great example of the V2 (verb-second) rule. Because the sentence starts with the adverb ibland (sometimes), the verb måste must take the second position in the sentence. The subject man is therefore pushed to the third position.

Why is it låta here instead of the present tense låter?

Because it comes directly after the modal verb måste (must). Modal verbs are always followed by the bare infinitive form of the next verb, which is why we use låta instead of låter.

Why is man translated as you here instead of one?

In Swedish, man is a generic pronoun used to talk about people in general, much like one in formal English. However, in natural English, we usually use the generic you (as in Sometimes you must...). It does not mean you the listener specifically, but rather anyone in that situation.