English has a relatively large number of auxiliary verbs (eg will, would, may, might, shall, should, must, ought) and verbal expressions (to be to: ‘we were to arrive at nine’; to have to: ‘we had to go’). Their main function is to express intentions or opinions (commands, possibility, etc). There is no straightforward match between these and their Spanish equivalents: Spanish has a rather smaller number of auxiliary verbs (eg deber, poder, querer) and verbal expressions (tener que hacer algo, haber de hacer algo). The major differences between Spanish and English are:

1. 1 Spanish auxiliaries may usually be used in all tenses (eg puede, podía, podría, ha podido, pudo, etc) while English auxiliaries have a maximum of two (which may in any case have independent modal values, eg may, might) and sometimes only one (eg must).

2. 2 In English, the Perfect auxiliary have is used only with the dependent infinitive (He may have done it), whereas in Spanish haber is used either with the dependent infinitive (podría haberlo hecho) or with the modal auxiliary (Habría podido hacerlo). There is a good deal of overlap in the values of modal auxiliaries in both Spanish and English: English It will be six o'clock or It must be six o'clock; Spanish Debes hacerlo or Tienes que hacerlo. Also, an auxiliary may have several modal functions: English can is associated with ability and possibility; Spanish deber with obligation and inference.

The following tables are presented according to categories of meaning rather than taking each modal auxiliary separately, and the equivalences given are necessarily rather vague.