Learn German: Wohin bist du gerannt?

The verb rennen (to run) is a mixed verb. This means it mixes rules: it uses the regular ge- and -t wrapped around the word, but its stem vowel changes like an irregular verb (the e becomes an a).

Why does the sentence use bist (are) instead of hast (have)?

In German, most verbs use haben (to have) to form the past tense. However, verbs that express a change of location or motion from A to B, like rennen, use sein (to be) as their helping verb. So literally, you are asking 'Where are you run to?'

Why use wohin instead of just wo?

The word wohin asks about a destination or direction (where to), which matches the motion of running to a new place. If you used wo, it would mean 'where at', asking in what fixed location the running took place (like running on a treadmill in a gym).

Why is gerannt at the very end of the sentence?

This is the standard word order for the German past tense. The helping verb (bist) takes the normal verb spot right after the question word, and the past participle (gerannt) gets pushed all the way to the end of the sentence.