Learn Polish: Dawniej czytywałem bardzo dużo, ale teraz nie mam czasu. - Formerly I used to read a lot, but now I don't have time.

While czytałem is the standard imperfective verb meaning 'I read' or 'I was reading', czytywałem is a frequentative verb. The -ywać suffix emphasizes that this was a regular, repeated habit over a long time. It translates perfectly to 'I used to read'.

Do I have to use frequentative verbs like czytywałem to talk about past habits?

No, they are usually optional. You can just use the regular imperfective czytałem, especially since the adverb dawniej already shows it happened in the past. Using czytywałem is a stylistic choice that adds a nostalgic or strongly habitual flavor to your sentence.

Could I use kiedyś instead of dawniej here?

Yes, kiedyś (once / sometime) would work and sounds very natural. However, dawniej specifically means 'formerly' or 'in the past' and sets up a strong, direct contrast with teraz (now). Also, unlike dawniej, kiedyś can also be used to talk about the future.

Why does the sentence use czasu instead of the basic noun czas at the end?