1. was drawing

His brother was drawing the house when I came in.

There is a specific time in the past, so use the past continuous tense.

2. was cooking

Yesterday afternoon, when I got home, my mother was cooking.

3. Were playing

Were they playing basketball at five o'clock yesterday afternoon?

4. were singing

This sentence seems to be missing a word. Behind the under

When I saw the twins, they were. . Sing below

5. were doing

What were the children doing at this time last night?

These are all past continuous tenses. Because there is a specific time in the past.