Learn French: Il demande si c'est normal que tu transpires pendant le yoga. - He asks if it is normal that you sweat during yoga.

The phrase c'est normal que (it is normal that) expresses a subjective judgment or opinion. In French, these types of expressions trigger the subjunctive mood for the verb that follows them (in this case, transpires).

tu transpires looks exactly like the regular present tense. How can I tell it's the subjunctive?

Because of the trigger c'est normal que, we know this is the subjunctive. For regular -er verbs like transpirer, the subjunctive forms for je, tu, il/elle, and ils/elles are identical to the normal present tense! To build it, you take the ils present tense stem (ils transpirent -> transpir-) and add the subjunctive endings (here, -es for tu).

Why is the first part Il demande si instead of using que?

Il demande si translates to "he asks if". When reporting a yes/no question, French uses si (if/whether) just like English does. If you used que, it would mean "he asks that" (which would be giving an order rather than asking a question).

Why is there a le in pendant le yoga? We don't say "during the yoga" in English.