"The man who is crying must be unhappy."
Translation:Ağlayan adam mutsuz olmalı.
The sentence seems to be drawing an inference: he is crying, so I conclude he is sad. I resisted using the -mali- suffix for this, since I don't mean that he has to be sad, he must be sad, he has a duty to be sad. I wished that the aorist olur could carry this weight somehow. Does -mali- also cover this kind of inference? Like, "I hear the church bells, it must be Sunday."
Zorunda can't replace the entire "olmalı"; it still needs a verb, and an infinitive at that:
"Ağlayan adam mutsuz olmak zorunda," is possible.
However, that would rather mean: The man who is crying has to/is obliged to be unhappy. He's bound by laws to be unhappy. ;p
"Zorunda" doesn't have the "assumption nuance" that -meli/-malı has.