Could you tell me If "to adults" is obligatory or not as in
I taught English to adults.
I was told in school that "to adults" is an indirect object.
I (S) + taught (V) + English (D.O) + to adults (I.O)
If that's obligatory, I would be attempted to call it as an adverbial complement modifying "taught".
I (S) + taught (V) + English (O) + to adults (Adverbial Complement)
But, I suppose "to adults" is just an adverb phrase. (not obligatory)
It seems to me that the removal of the prepositional phrase can't render the sentence ungrammatical. i.e., "I taught English." I perfectly understand.
I (S) + taught (V) + English (O) + to adults (Adjunct)