Learn German: Wenn du das Gerät ausprobierst, hilft dir der Trainer. - If you try out the machine, the trainer will help you.

You are right that separable verbs usually split in normal sentences. But here, the clause starts with Wenn (if), making it a subordinate clause. In a subordinate clause, the conjugated verb is pushed to the very end. When a separable verb goes to the end, its parts come back together into one single word (ausprobierst).

Why does the second half of the sentence start with the verb hilft instead of der Trainer?

In German, the verb must be in the second position of the main clause. The entire first part of the sentence (Wenn du das Gerät ausprobierst) acts as a single block in position one. To keep the verb in position two, hilft has to come right after the comma, before the subject (der Trainer).

Why does the sentence use dir instead of dich for 'you'?

The verb helfen (to help) always requires its object to be in the dative case. Because of this, you must use the dative pronoun dir instead of the accusative dich.

Can Wenn mean both 'when' and 'if'?

Exactly! Wenn is used for conditions (if) and also for time (when or whenever, for present or future events). In this sentence, it can translate to either 'if you try out the machine' or 'when you try out the machine' because the meaning is very similar.