Suppletion is a term used in morphology when the relationship between morphemes cannot be shown through a general rule. For instance in English, suppletion describes the relationship between the present ~ past-tense of ‘go’ ~ ‘went’, or the comparative form ‘better’ ~ ‘good.’

Persian uses a whole host of suppletive morphemes (see also the subsections dealing with the past and the present tense in the Verbs section). But two of the most essential to recognize are:

For the infinitive بودن budan ‘to be’:

the present hastam هستم
the past budam بودم

For the infinitive دیدن didan ‘to see’:

the present stem bin- بین
the past stem did- دید