The imperfect tense in Welsh can translate as the following. For example, Roeddwn i'n gweithio can translate as 'I was working' or 'I used to work'. Note there is a material difference between 'I worked' and 'I was working': in the first phrase, the action has been completed in the past (preterite or past perfective tense), whereas in the second, the action is continuous, yet still in the past (hence, imperfect or past imperfective tense).

Imperfect Tense - bod (to be)
English Cymraeg English Cymraeg
I was, I used to Roeddwn i We do, We are Roedden ni
You were, you used to Roeddet ti You were, you used to Roeddech chi
He was, he used to Roedd e/o They were, they used to Roedden nhw
Use the particle 'yn' to connect Verb-Nouns. Yn shortens to 'n after a vowel, as in the following examples.

Roeddet ti'n nofio ddoe.

You were swimming yesterday

Roedd hi'n caru Sarah.

She was loving Sarah.

Roedden nhw'n sefyll tu allan y dafarn.

They were standing outside the pub.

Remember: If using the particle with a noun or adjective, the noun must undergo a soft-mutation if possible.

Roedd y bocs yn las.

The box was blue.

Roeddwn i'n feddyg.

I was a doctor.

Imperfect Tense Responses bod - to be
English Cymraeg English Cymraeg
Yes: I do, I am Oeddwn / Nac oeddwn Yes: We do, We are Oedden / Nac oedden
Yes: You do, you are oeddet / Nac oeddet Yes: You do, you are oeddech / Nac oeddech
Yes: He does, He is oedd / Nac oedd Yes: They do, They are oedden / Nac oedden
Remember: Welsh does not use 'yes' or 'no' responses. Instead, you would answer with a more direct 'I was' or 'He was' type response.

Doeddet ti ddim yn nofio ddoe.

You weren't swimming yesterday

Doedd hi ddim yn caru Sarah.

She wasn't loving Sarah.

Doedden nhw ddim yn sefyll tu allan y dafarn.

They weren't standing outside the pub.

Doedd y bocs ddim yn las.

The box wasn't blue.

Doeddwn i ddim yn feddyg.

I wasn't a doctor.