In some cases the same word can be adverb, conjunction and preposition and sometimes this distinction is not made in English. The conjunction is with que, the preposition with de and the adverb stands alone.
antes que = before (He did it before I could intervene.)
antes de = before (He did it before six o' clock.)
antes = before (He did it before.)
depois que = after (He did it after he had been informed.)
depois de = after (He did it after six o' clock.)
depois = later (He did it later.)
Decide in the following sentences whether a conjunction, a preposition or an adverb is to be used. To understand the structure of the sentences in detail, it is useful to remember what has been said in consecutio temporum and in the chapter about the conjuntivo. Furthermore it is useful to remember what has been said in the chapter infinitivo pessoal. In other words, the sequences of tenses has an impact as well in subordinated clauses introduced by a conjunction and in order to understand the difference between a verbal noun and an finite form we should understand that a verbal noun can only show up after a preposition and finite verb only after a conjunction.