We have already used a lot of conjunctions without explicetely explaining them. In most cases this is not necessary either, because at least the simple ones can be understood without any further explanation. The conjunction introduces, if we put aside the copulative conjunctions, a subordinate clause and establishes a relationship between the action / events described in the main clause and the action / events described in the subordinate clause. The meaning of a sentences can change radically depending on the conjunction used. (Or becoming complete nonsense.)
He doesn't make any sports, although he is ill.
He doesn't make any sports, because he is ill.
He doesn't make any sports, when he is ill.
He doesn't make any sports, even if he is ill.
To a lot of conjunctions exists a corresponding preposition. We have already seen that this is relevant, because only in the case that there is prepositions that corresponds to the conjunction a shortening of a subordinate clause with a infinitivo (pessoal) is possible, see chapter 18. The preposition requires a noun or something that is equivalent to a noun, a pronoun or a verbal noun and the only verb form in Portuguese that can have the syntactical function of a noun it the infinitivo (pessoal).
He went away without saying anybody where he goes.
He went away without him.
More then in other languages there is a tendency in Portuguese to shorten subordinate clauses with an infinitive. This is the case, because with the infinitivo pessoal it is possible to shorten a subordinate clause with an infinitive even in the case that subject of the infinitivo (pessoal) differs from the subject of the main clause. It is therefore useful for a deeper understanding to see that the futuro do conjuntivo and the infinitivo pessoal have completely different syntactical functions, although there is no difference in the form, see chapter 16 and chapter 18.
In most cases the preposition distinguish itself from the conjunction by the que. The conjunction has a que and the preposition not.
Depois de algum tempo você aprende a diferença.
After a certain time, you will understand the difference.
Depois que eu vi como ela dança.
After I saw her dancing.
Até ele se deu conta.
Even he realized that.
Devemos continuar trabalhando até que o consigamos.
We have to continue working until we succeed.
Estamos esperando aqui desde as 12 h.
We have been waiting here since 12 o'clock.
Desde que se tornou independente, em 1958, a Guiné só conheceu regimes ditatoriais.
Since his independance in 1958 Guinea was alway governed by a dictator.
To the general rule that in the preposition the que of the conjunction is substituted by a de (depois que => depois de) there is obviously an exception, otherwise it would be boring. The conjunction corresponding to the preopositon a fim de is a fim de que. In this case que is added.
If someone finds it difficult to understand why the infinitivo (pessoal) is a verbal noun and can have the syntactical function of a noun there is an easy way to see that. The infinitivo (pessoal) can be substituted by any noun.
subordinated clause shortened by an infinitive:
Depois de ter comido, vamos a praia.
After having had dinner, we go to the beach.
infinitive substituted by a noun:
Depois do almoço vamos a praia.
After dinner we go to the beach.
Ter comido abd almoço have the same syntactical functions.
Concerning the mood to be used is always valid the general rules. If the conjunction or the subordinate clause introduced by this conjunction expresses a finality, an intention, hope, fear, non reality, etc. in other words if it expesses a subjective attitude towards the world we use the conjuntivo. The same rules we have already seen in the verbs are to be applied, see chapter 12. The sequence of tenses is to be respected, see chapter 14. In other words it has to made clear whether the action / event described in the subordinated clause happened, happens or will happen before, at the same time or after the events described in the main clause, see chapter 14.
Between the conjunctions some are not problematic, there is a one to one translation in English and they have the same semantic value as their English counterparts. The focus of this chapter is on the conjunctions that are for different reasons more complicated.