I am not a native speaker so it's kind of difficult for me to decide which one is correct:
I usually stay in bed long in a bright summer morning.
I usually stay in bed long on a bright summer morning.
I may be mistaken, but to my mind "on" sounds better here. Though I'm not sure how to explain it to my pupils. Is there a rule concerning such cases? Help me, please!
Yes, you are right."on" will be used in this sentence. "In" is used only when there is definite article (the) before "morning".Moreover, If any other adjective is used before or after "the", we will use "on" e.g.
He met me in the morning
I will leave on the Sunday morning.
He met me on a pleasant morning.