He invited Mr. Smith to the party that day.
He had invited Mr. Smith to the party that day.
Please tell me the difference and which of the two is grammatical?
He invited Mr. Smith to the party that day.
He had invited Mr. Smith to the party that day.
Please tell me the difference and which of the two is grammatical?
Both are grammatical though they mean different things. (Note that "party" used as a verb is much rarer than "party" used as a noun so both sentences are unusual.)
The first sentence simply says that the invitation was made at some past instant.
The second sentence says that the invitation was made at some past instant that preceded some implicitly understood past event.