Student or Learner
I am too bad in these prepositions: In, on, at.
I don't know when to use them.
"I didn't let you in/on/at beach." <------ Must I use IN, ON, or AT in this sentence?
If the OP's job is to guard the entrance to a beach, then that's fine. Otherwise, I can't see how anyone could not let someone else on the beach. I wondered if perhaps it should have been "I didn't let you go to the beach" (perhaps what a parent says to a child).
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.