Learn Hardcore German: Bevor ich trainiere, dehne ich meine Muskeln vorsichtig. - Before I train, I stretch my muscles carefully.

In German, any clause introduced by a subordinating conjunction like Bevor follows the verb-final rule: the finite verb moves to the very end of that subordinate clause.

Why is there a comma after Bevor ich trainiere?

German requires a comma to separate a subordinate clause (here starting with Bevor) from the main clause. This holds whether the subordinate clause comes first or second.

Why do I say meine Muskeln instead of a reflexive pronoun like mich?

Here dehnen is used as a transitive verb taking die Muskeln (accusative plural) as its object. If you used a reflexive construction, you’d say ich dehne mich, which is also correct but focuses on the whole body rather than specifically on the muscles.

Is it correct to say Ich dehne mich vorsichtig instead of meine Muskeln?

Yes, ich dehne mich vorsichtig (using the reflexive sich dehnen) is perfectly acceptable. It’s just a slightly different nuance: meine Muskeln specifies what you stretch; mich makes it more general.

Why is the adverb vorsichtig at the end of the sentence? Can I move it?

Adverbs of manner like vorsichtig usually follow objects in German. You could say Ich dehne vorsichtig meine Muskeln, but placing vorsichtig at the very end is more idiomatic and emphasizes the carefulness.

Can I swap the clauses and say Ich dehne meine Muskeln vorsichtig, bevor ich trainiere? Do I still need the comma?

Absolutely. When the main clause comes first, you still separate the subordinate bevor-clause with a comma:
Ich dehne meine Muskeln vorsichtig, bevor ich trainiere.

Could I use a noun phrase like Vor dem Training instead of Bevor ich trainiere?

Yes. Vor dem Training dehne ich meine Muskeln vorsichtig is an equivalent alternative. It uses a prepositional phrase (Vor + dative) rather than a subordinate clause.

Why is the present tense used instead of a future tense here?

German often uses the present tense to describe imminent future actions when the time relation is clear. So Bevor ich trainiere, dehne ich ... naturally refers to a future training session without needing a future tense.