Correct usage of Diventare

Question

I have a problem understanding the correct usage of diventare. During a recent lesson my (online) teacher had me watch a video during which a bear became angry. She then asked me to describe it. One of the things that I said was:

L’orso diventa molto arrabbiato

She told me that I could not use "diventare" that way but could offer me no guidance on why other than "It's not Italian".

If one believes the ever-unreliable Google translate then I can become a doctor ("divento un medico"), he can become a doctor ("diventa un medico"), I can become angry ("divento arrabiato") but I will admit that if I enter "he becomes angry" it goes for a reflexive ("lui si arrabbia").

I'm not saying that she's wrong but can someone help me get my head around WHY this is? Because without understanding it, I'm likely to misuse it again since diventare is too useful a verb to NOT use regularly.

Answer 1

It's a matter of idiomatic vs. grammatical. You might say, without violating any grammatical norm, any of those phrases (divento/a medico, divento/a arrabbiato), but none of them are idiomatic, that is, none of them would spontaneously be pronounced by an Italian. You'd say, respectively, mi laureo in medicina (or other steps to actually become a doctor) and mi arrabbio.

The use of diventare is more restricted than that of “to become”, almost only to transformations: Il bruco diventa farfalla and the like.