Although it is acceptable to use the simple past for all instances where you talk about past events and states, it is more idiomatic to use certain auxiliary constructions instead, in a number of circumstances. These constructions consist of putting a helper verb in the past tense in front of the infinitive you want to modify. Whilst in many languages, being able to talk about somethingyou were doing, used to do or had done requires learning new tenses, all that you need to do in Celinese is remember to stick an auxiliary verb (or prefix) in front.
I was working: foí air caithí/air caithí foí — Known as the imperfect, or the past continuous, when you discuss things that you were doing, a very similar construct is used to when one talks about things one is doing, the present continuous. The present continuous is formed by air or ag and the infinitive of a verb either in front of or directly after a fully conjugated sí (to be) - ceoním, we speak, becomes sím air ceoní or air ceoní sím. we are talking. To turn that to 'we were talking', you simply put sí into the past tense: foím air ceoní or air ceoní foím. Take athlí, to live; foís air athlí/air athlí foís is 'you were living.'
I used to work: caithí tynðoí — The past habitual, describing things you once used to do, is simpler still. You take tynðí, which means to usually or to tend to, put it in the past and place it in front of your verb - so one literally says, 'I tended to work.'
I had worked: rogaithloí — this form, known as the pluperfect, is used to narrate actions or states that are more remote in the past to verbs in the simple past or imperfect. It is made simply by prefixing "ro-" - short for "after" - in front of a verb in the past tense, and mutating the initial consonant if necessary. An example: Naint anðí né ronoroí, mair sé lyroí: I had not wanted to go there, but I enjoyed it. This is losing out to the simple past in common parlance.
Other than the past habitual, which is always marked, these other past tenses can be dropped if you choose. Some parts of the Celinophone world use these distinctions more than others.