present tenses in subclauses beginning with time conjunctions
They include: while; when; by the time; untill; as soon as; once; before; after
I won't believe it untill I have seen it.
have another drink before you go.
She'll go on holiday after she has passed her exams
time conjunctions: they link main clause and subclause
A main clause is a complete thought that can stand alone as a sentence.
A subclause: Needs a main clause to be complete, starts with a subordinating conjunction (e.g.because,although,when, if)