Can someone explain why it is "in dem" here? thanks.
Because in this case, 'in' is a dative preposition so 'Restaurant' takes 'dem.' If it was being used to indicate that someone was going into the restaurant, then it would be accusative and it would be 'in das' (e.g.- Er geht ins Restaurant).
This means the same: Den Mantel gibt es nicht im Restaurant.
That doesn't sound natural.
Couldn't it be "Der Mantel ist im Restaurant nicht"?
Yes, but it's not the normal word order.
Could it not be "Der Mantel ist nicht am Restaurant" since the clue says "at the Restaurant"?
No, "am Restaurant" implies that the coat is somewhere near the restaurant, but not inside the restaurant. That doesn't make a lot of sense.
Hmm, I thought "am" meant "at the," or that's at least one of its meanings. I have no trouble using "im" if that is not the case.
It didn't accept "beim Restaurant." Wouldn't that be a better translation of "at the"? 'Im' would mean "in the," wouldn't it?