Learn Swedish: Han fattade aldrig varför hon grät. - He never grasped why she was crying.

Both mean to understand, but fatta is more colloquial and informal, much like saying to get it or to grasp in English. Förstå is the standard, neutral word. Fatta can also literally mean to catch or take physical hold of something.

Why is it translated as was crying instead of just cried? Does Swedish have an ongoing action form?

Swedish does not have a direct grammatical equivalent to the English continuous form like was crying or is doing. The simple past tense grät covers both cried and was crying. You just rely on context to pick the most natural English translation.

Is grät an irregular verb?

Yes, it is the irregular past tense of gråta (to cry). It is a strong verb, meaning it changes its core vowel rather than adding a standard past tense ending. The forms to memorize are gråta (infinitive), gråter (present), grät (past), and gråtit (supine).

If we wanted to say why she was NOT crying, where would inte go?

The phrase varför hon grät is a subordinate clause. In Swedish subordinate clauses, adverbs like inte or aldrig must come right before the verb. So it would become varför hon inte grät. Notice how this contrasts with the main clause Han fattade aldrig, where the adverb comes after the verb!