Breakdown of Wenn der Mann verletzt ist, müsste der Krankenwagen kommen.
sein
to be
der Mann
the man
kommen
to come
müssen
to have to, must
Questions & Answers about Wenn der Mann verletzt ist, müsste der Krankenwagen kommen.
Why do we use müsste here instead of muss?
müsste is the Konjunktiv II form of müssen (must / to have to). While muss means something definitely "has to" happen, müsste means it "would have to" happen. It is used here because the situation is hypothetical—we are stating what would be necessary if the man turns out to be injured.
Why does the second half of the sentence start with müsste instead of der Krankenwagen?
This happens because of the "verb second" rule in German. The entire "if" clause (Wenn der Mann verletzt ist) counts as the first block of information (position 1). To keep the main verb in position 2, müsste must come right after the comma, pushing the subject (der Krankenwagen) to the other side.
Why is ist at the very end of the first part?
The word wenn (if) is a subordinating conjunction. Words like this act as "kickers" that push the conjugated verb—in this case, ist—all the way to the end of their clause, right before the comma.
Could I use ob instead of wenn for "if" in this sentence?
No, ob is only used for indirect yes/no questions (for example: "I don't know if he is injured"). When you are setting up a conditional statement like this one ("If X is true, then Y would happen"), you must always use wenn.