If a sentence is introduced by a preposition only the infinitivo or the infinitivo pessoal is possible, because in Portuguese only these two verb forms are verbal nouns and can have the syntactical function of a noun. In general the conjunction has a que and the preposition not. If the subject of the action / event described by the infinitivo (pessoal) differs from the subject of manin clause, the infinitivo pessoal is compulsory. Not because it would be grammatically incorrect, but because the meaning of the sentence would change, see 16. Equally the infinitivo pessoal is to be used, if the subject is explicetly mentionned.
It is useful to see that in regular verbs there is no differenct between the futuro do conjuntivo, that can be used in a subordinate clause, and the infinitivo pessoal, which is used after a preposition.
Furthermore the sequence of time, see 14, is to be respected. We use the compound forms of the infinitivo (pessoal) in case of anteriority and the simple forms in case of simultanity or posteriority.
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