Learn Hardcore Dutch: Tijdens de training fluit de scheidsrechter als iemand stopt. - During the training the referee blows the whistle when someone stops.

In Dutch main clauses you have V2 (verb-second) word order. That means the finite verb must be the second element. Here tijdens de training is your first element (an adverbial phrase), so the verb fluit comes next, then the subject de scheidsrechter. If you start with the subject, there’s no inversion:
De scheidsrechter fluit tijdens de training als iemand stopt.

Why does the subordinate clause als iemand stopt come at the end of the sentence?

The conjunction als introduces a dependent (subordinate) clause in Dutch. In subordinate clauses the finite verb goes to the very end. That’s why stopt appears after the subject iemand in als iemand stopt.

What does the preposition tijdens mean, and when do I use de or het after it?

tijdens means during. It’s always followed by a noun phrase with its article. You choose de or het based on the noun’s gender:

de for common-gender nouns (e.g. de training)
het for neuter nouns (e.g. het feest)
Hence tijdens de training but tijdens het feest.

Why is the article de used for scheidsrechter and not het?