What is a relative clause?
There are many kind of clauses.
A relative clause is a clause, which gives more information about a noun in a sentence.
Ex:
There is the boy who won the race
I remember a time when we weren’t so busy
Relative clause of time
Adverbial clause
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It is a dependent/subordinate clause acts as an adverb.
It has a subject and a verb of its own
Starts with an adverb of time
If it is at the beginning of the sentence, a comma is required
If it is at the last part of the sentence a comma is not required