While I (ente) the room ... I (do) my lessons

While I entered the room I was doing my lessons. — Пока я вошел в комнату, я делал уроки.

I was doing my lessons while I entered the room. — Когда я вошел в комнату, я делал уроки.

The first part of the sentence is in Past Indefinite (entered) and the second part is in Past Continuous (was doing).

Past Indefinite is used for a past action. Past Indefinite is formed with the help of the suffix –ed for regular verbs.

Past Continuous is used for past action which is in progress at point of time in the past. Past Continuous is formed — was / were + verb+ing.