Breakdown of Der Trainer sieht, dass du Muskeln aufbaust.
du
you (informal singular)
sehen
to see
dass
that
der Trainer
the trainer / the coach
Questions & Answers about Der Trainer sieht, dass du Muskeln aufbaust.
Why is the verb aufbaust all one word at the end of the sentence?
The verb here is aufbauen (to build up), which is a separable verb. Normally, the prefix auf separates and goes to the end of the sentence (like in Du baust Muskeln auf). However, because this sentence uses dass (that), which starts a subordinate clause, the conjugated verb is pushed to the very end. When a separable verb is pushed to the end like this, the prefix and the verb stick back together into a single word: aufbaust.
Could I just say dass du Muskeln baust without the auf?
Not quite. While bauen means 'to build' (like building a house), aufbauen means 'to build up' or 'develop.' In German, you 'build up' muscles (Muskeln aufbauen), so you need the separable prefix auf to make it sound natural.
Is the comma before dass always required?
Yes! In German, you must always put a comma before a subordinate clause. Since dass (that) introduces a subordinate clause and changes the word order, you need the comma to separate it from the main clause (Der Trainer sieht).
The English translation says 'are building', but why isn't there a word for 'are' in the German sentence?
German doesn't have an '-ing' continuous tense like English does. The single present tense verb aufbaust covers both 'you build' and 'you are building'. You never translate the 'are' directly in these cases.