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?

Dutch nouns are either common gender (taking de) or neuter (taking het). Scheidsrechter (referee) is a common-gender noun, so it takes de. Unfortunately there’s no hard rule that covers all nouns—you generally have to memorize which article goes with each word or learn typical patterns.

Why is stoppen conjugated as stopt in als iemand stopt?

Because iemand (“someone”) is a third-person singular subject, you form the present tense by adding -t to the verb stem. So stop- becomes stopt.

Could I use wanneer instead of als here?

Yes. wanneer iemand stopt also means when/whenever someone stops. In everyday language als is more frequent for repeated or conditional events, while wanneer can sound a bit more formal or refer to a single occasion. Both are correct.

What happens to the word order if I start the sentence with the subordinate clause Als iemand stopt?

When you begin with Als iemand stopt, you still apply V2 in the following main clause. You’d write:
Als iemand stopt, fluit de scheidsrechter tijdens de training.

Do I need a comma before als iemand stopt?

In Dutch, commas before subordinate clauses are optional, especially if the clause is short. Many writers omit it:
Tijdens de training fluit de scheidsrechter als iemand stopt.
You can add a comma for clarity or style, but it’s not mandatory.