The Breton verb, whose conjugations are notoriously known for the way in which they vary according to the syntactic structures and interlocutive interplays in which they are inserted, displays remarkably original phenomena in the field of tense sequence. Firstly, the Breton verb has two simple tenses, the potential and the unreal, both of which can be used in simple sentences as well as in complex sentences, in the main and/or subordinate clauses, with uses analogous to those of romance subjunctives and conditionals; secondly, the tense of the subordinate verb, which is usually determined by the meaning of the verb in the main clause, displays a whole range of fluctuations which are determined by the presence of other modal indicators such as negation and the imperative. In this study, it is shown that in classical Breton there is a competition between a modal sequence in present contexts and a temporal sequence in past contexts, with the latter acquiring the variations of modal sequence in present-day Breton on account of the possibilities offered by the tense system.