This article discusses the phonological and analogical developments of the inherited subjunctive/desiderative suffix *-āse/o- in the Brittonic languages, which formed the Welsh and Cornish present subjunctive and the Breton future. It is demonstrated that, once the treatment of intervocalic *s > *h is understood, many of the forms of the future/present subjunctive can be explained by regular sound changes. Middle Breton is more conservative than Middle Welsh in preserving h only in the plural endings: Welsh generalized the characteristic plural h into the singular endings as well. The verb 'to be' differs from the regular verb both in reflecting originally separate subjunctive and desiderative stems, and in tending to have the British accent on its initial syllable. As a result of sound change and the different developments of the verb 'to be', allomorphy within the future/present subjunctive paradigms and between 'to be' and other verbs was extreme, and this led to a large number of by-forms created by paradigmatic levelling.