English has a relatively large number of auxiliary verbs (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, possibilities, etc.). There is no straightforward match between these and their equivalents in French which, like Spanish and Italian, has a rather smaller number of auxiliary verbs (e.g., devoir, pouvoir, vouloir). The major differences between French and English are:

1. French auxiliaries may normally be used in all tenses and moods (e.g., Je peux/puis, j'ai pu, j'avais pu, je pus, je pourrai, j'aurai pu, j'aurais pu, je puisse, j'aie pu), while English auxiliaries have a maximum of two (may, might), and sometimes only one (must).

2. In English, the perfect auxiliary have is used only with the dependent infinitive (He may have done it), whereas in French avoir is used either with the dependent infinitive (Elle a pu être vexée = She could have been upset) or with the modal auxiliary (Elle peut avoir été vexée). There is a good deal of overlap in the values of modal auxiliaries in both French and English, but often they do not have equivalents.