Past Tense or Past Perfect?

When she ____ on the course, she had never studied a foreign language before

• had enrolled
• enrolled

She looked really sad, but I didn't know what

• happened

• had happened

_____ when you rang the doorbell?

• Did she already leave

• Had she left

When I closed the door, I realised that I ____ my keys inside.

• had left

• left

By the time I moved in, they ____ the building work.

• had finished

• finished

I looked in the letter box yesterday and the letter still ____

• didn't arrive

• hadn't arrived

I opened the door, and ___ inside.

• had gone

• went

This is the oldest building in the town. It ____ over 200 years ago.

• was built

• had been built