Except in languages with zero copula, to be is one of the most useful verbs, and in Celinese, it is no exception. It is conjugated regularly in the present tense, taking the same verbal endings as the majority of verbs in modern SEC: sí, sís, sío, sím, síot, sín. In the past tense, thanks to suppletion, we instead use the verb fí (to make) in the past tense - foí, foís, foío, foím, foíot, foín (as a result, fí itself has an irregular past form, foðoí, etc.)
Like í (to have) and a few other verbs, sí has an alternate way to express the future and conditional tenses other than the typical use of fyðí and weðí periphrastically respectively: a clipped fy- and we- is added to conjugated present tense forms of sí. Thus, we are, sím, becomes fysím (we will be) or wesím (we would be.) One sometimes sees these forms combined with the past tense, as in wefoím (we would have been), but this is somewhat uncommon.
Sí can also be used with either air or ag and a second verb in the infinitive to create a continuous form of the present or past. The second verb can be placed before or after sí, as in air ionelí sím or sím air ionelí (we're arriving); ag ceoní foí or foí ag ceoní (I was speaking.) It is, however, equally valid - and perhaps more common - to use the simple present or past to refer to continuous actions or states, with the specifically continuous forms most used for disambiguation purposes.