Imperfect and preterite

Imperfect and preterite
 

Yo estaba cantando cuando llegó.

I was singing (past - imperfect) when he arrived (also past but shorter time than me singing - preterite).

Correct? Any reason other than length of time?

The past imperfect wasn't used in your example. You used the past progressive (the past imperfect of 'estar' (estaba) + gerundio (cantando)). To use the past imperfect, just conjugate cantar into the imperfect tense.

Yo cantaba cuando él llegó.
I was singing when he arrived.

The past imperfect is used to 'set the stage' for another event that occurred. You were doing something when it was interrupted by another event.

Thanks for a good lesson! After reading my textbook several times I finally understood your post. It was an aha moment when I finally got it.
As you plainly said.......I had the right idea but was conjugating the wrong word.

You're welcome.

Just so you understand, it's perfectly fine to use 'estaba cantando' in your sentence, so long as you realize that it is a progressive form. Your thread title had me convinced that you were studying the imperfect and preterite tenses (not their progressive forms). Two words participate in the progressive form - a conjugated form of estar, plus a gerundio (present participle). This page may help you to understand the difference between the progressive forms and the 'simple' forms.

Preterite
estudié (studied - event completed in the past)

Preterite Progressive
estuvo estudiando (was studying, but it had an end)

Imperfect
estudiaba (was studying, event started but not necessarily ended in the past)

Imperfect Continuous (Progressive)
estaba estudiando (was studying, event started and continued in the past)

You were correct on the tenses.
 

Indeed the study was for preterite and imperfect.

Thanks for the corrections and the link provided. The link has many good topics for learning grammar and verb usages. At this stage I believe I am learning (relearning) more English than Spanish. Guess it will all work out later.