19.7.2 caso if
The conjunction caso is a synomym for se (if), although more common is se. However there is a big difference, that is a little bit hard to understand, in other words, it is not very consistent. We have seen in 12.2.5 that the futuro do conjuntivo is used when there is no doubt that the event is going to happen in the future, but the moment it will happen is uncertain. In this case, in opposite to other roman languages where something like a futuro do conjuntivo doesn't exist, in such a context in Portuguese the futuro do conjuntivo is used and not the presente do conjuntivo as in other roman languages.
A conditional clause introduced by the conjunction se requires the futuro do conjuntivo, see 19.7.1. One can try to explain that, although it doesn't fit with the rules mentioned in 12.2.5. A conditional clause establishes an objective relationship of the type if A is given, B will follow. This is not an subjective evaluation of the reality that would require the presente do conjuntivo, this is an objective relationship. (At least the speaker believes that.)
Caso however requires, as in any other roman language, for instance in Spanish, "En el caso de que haga buen tiempo, voy a la piscina" <=> "If the weather is fine, I go swimming", the presente do conjuntivo and this is without any doubt inconsistent and has to be accepted as it is. It doesn't make any difference concerning the meaning, but in one case we use the futuro do conjuntivo and in the other case the presente do conjuntivo. The system is therefore instable and leads to discussions.
| se => futuro do conjuntivo caso => presente do conjuntivo |
futuro do conjuntivo: Se amanh� fizer sol, irei para a piscina.  |
| presente do conjuntivo: Caso amanh� fa�a sol, irei para a piscina. |
| If the weather is fine tomorrow, I go to the swimming pool. |
The difference between the presente do conjuntivo and the futuro do conjuntivo is very subtle, perhaps too subtle, and that explains the inconsistency and why the futuro do conjuntivo disapeared in Spanish. From a pragmatic point of view the rule is this: Caso requires the presente do conjuntivo, se requires the futuro do conjuntivo although there is no difference in the semantic value.
Acaso means by incidence an requires the futuro do conjuntivo. It can be combined with se.
| se combined with acaso |
| Se acaso fizer sol, irei para a piscina. |
| If by incidence the sun shines tomorrow, I will go to the swimmingpool. |
| caso if / provided |
| Caso voc� v�* ao mercado, compre acerola. |
| If you go to the market, buy acerola cherries.** |
| Caso seja envolvido num acidente rodovi�rio deve identificar-se. |
| If you get involved in a traffic accident you have to identify yourself. |
| Caso trabalhe mais de 5 horas consecutivas, o funcion�rio tem direito � uma pausa. |
| If an employee works more than five hours without interruption, he has the right for a break. |
* Complete irregular conjuntivo do presente: v�, v�s, v�, vamos, va, v�o.
** Acerola, is a fruit that looks like a cherry, but is actually a completely different fruit.
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