As we have already said in general little attention is given in grammar books to the preposition, conjunctions and adverbs. They are considered minor problems. If you take a closer look at them you will see that their use is complicated. The difficulties are due to three problems.
Very often one language doesn't distinguish between preposition and conjunctions, but there is a difference in the other language. Therefore, one must be able to distinguish between the preposition and the conjunction.
Sometimes a criterion is to be taken into account in one language, but not in the other one, in other words, the translation is to be made with a different conjunction depending on the context.
In romance languages some conjunctions that express insecurity, nonexistence or a personal / emotional attitude towards the world requires the subjunctive and sometimes a conjunction requires the subjunctive or the indicative depending on the context.