Breakdown of Du siehst heute gut aus.
du
you (informal singular)
gut
good / well
heute
today
aussehen
to look (appear)
Questions & Answers about Du siehst heute gut aus.
Why is aus at the very end of the sentence?
This is because the verb here is aussehen (to look / to appear). It is a separable prefix verb. In a normal main sentence, the core part of the verb (siehst) takes the second position, and the prefix (aus) gets chopped off and pushed to the very end.
Wait, shouldn't the du form of sehen be "sehst"? Why is it siehst?
The base verb sehen (to see) is irregular. For the du (you) and er/sie/es (he/she/it) forms, the stem vowel changes from e to ie. Because aussehen is built on sehen, it follows the exact same pattern: du siehst ... aus.
Does aussehen mean "to look at" something?
No, aussehen specifically means "to look" in the sense of appearing a certain way, like looking good, looking tired, or looking like your brother. If you want to say "to look at" something, you would use a different verb.
Why does heute come before gut? Could I say "Du siehst gut heute aus"?
It sounds much better this way. In German, words that tell us when something happens (time words like heute) usually come right after the verb and before words that tell us how it happens (manner words like gut).