Learn Hardcore German: Der Arzt sagt, Sport sei besser als Medizin, wenn man fit bleiben will. - The doctor says sports are better than medicine when you want to stay fit.

Because this sentence is an example of indirect (reported) speech in German. In reported speech you normally use Konjunktiv I to indicate that you’re reporting someone else’s words rather than stating a fact yourself. The indicative form would be ist (direct speech), but in reported speech you switch to sei to show that the doctor said it.

Why is there no dass after sagt, and why is there a comma instead?

In German you have two main ways to introduce a subordinate clause after verbs like sagen:

With dass: „Der Arzt sagt, dass Sport besser ist als Medizin.“ (indicative)
Without dass, using a comma and Konjunktiv I: „Der Arzt sagt, Sport sei besser …“
Both are correct. Omitting dass is more concise and common in formal written German when you use the subjunctive form.

Why does the verb will go at the end in wenn man fit bleiben will?

Because wenn introduces a subordinate clause. In German subordinate clauses the finite verb always appears at the end. So you naturally get …, wenn man fit bleiben will.

Why is Sport used without an article?

In German, activity or discipline nouns (like Sport, Musik, Kunst) are often used without an article when speaking in a general sense. You’re not referring to a specific sporting event or piece of equipment, but to the concept of exercise in general, so no article is needed.

Why is als used instead of wie in besser als Medizin?

Standard German uses als to introduce the second element in a comparison with a comparative adjective/adverb. You say besser als (better than). Using wie here would be incorrect in standard German (although you might hear it in some dialects).

Why is the pronoun man used here? Could you say du or wir instead?

man is the indefinite pronoun “one” or “people in general.” It’s used when you want to make a general statement about anyone. If you said du, it would address the listener directly (“if you want to stay fit”), and wir would include the speaker (“if we want”). Both are grammatically fine but change the nuance from a general truth to a direct or inclusive statement.

Could you use wäre (Konjunktiv II) instead of sei, and what difference would that make?

Yes, you could say Der Arzt sagt, Sport wäre besser als Medizin, using wäre (Konjunktiv II). In reported speech, though, Konjunktiv I (sei) is preferred if its form differs from the indicative. Konjunktiv II can imply doubt or a hypothetical situation. Here, using wäre might make the statement sound more speculative, whereas sei simply reports the doctor’s actual claim.