Time clauses are used to show when an action happens in relation to another action. They have a main clause and a subordinate clause, with the subordinate clause indicating time using conjunctions like when, while, before, after, until/till. The subordinate clause can use present, present perfect, or past tense depending on the tense of the main clause. Adverbs like when, while, before, after, until/till indicate whether the action happened simultaneously, previously or subsequently to the other. If the conjunction is when, while, before, after, the subject of the main clause and subordinate clause can be the same and the subordinate clause can be replaced by a -ing form.