Learn Swedish: Att haja till är bara en reflex.

On its own, the slang word haja usually means to get it or to understand. However, when combined with the aspectual particle till, it forms the phrasal verb haja till, which means to startle, to jump, or to flinch. The particle till gives the action that sudden, involuntary, momentary quality taught in this lesson.

Where does the stress fall when pronouncing haja till?

As with almost all Swedish phrasal verbs, the stress falls heavily on the particle. You must pronounce it as haja TILL. If you stress haja instead, a native speaker might not realize you are using a phrasal verb at all.

Why is the sentence structured as Att haja till är...?

Here, the entire infinitive phrase att haja till acts as the subject of the sentence. Swedish uses this att plus infinitive construction just like English uses 'to' plus a verb or an -ing word (like 'startling') to turn an action into the subject of a sentence.

Is haja till used for scaring someone else, or for being startled yourself?

It is strictly used for being startled yourself, describing your own sudden, physical reaction. If you want to say that you startled someone else, you cannot use haja till; you would use a different verb entirely, such as skrämma.