what is the 'past perfect simple'?

'They had left by the time I got home.'

When do we use the ‘past perfect’?

When we talk about the past we use the past simple tense:

'He came home.'

When we want to talk about something that happened before this we use the past perfect tense:

'I had finished before he came home.'

The past perfect simple is: had + past participle:

'By the time Sue went shopping, the shops had closed.'

'Bill Gates had made millions of dollars before he was 30.'

'The car was clean. They had washed it every weekend.'

'My family had had that dog for years.'

What’s the difference between the ‘past perfect’ and ‘present perfect’?

The present perfect is connected to the present time.

'I have never been to Canada.'

The past perfect is connected to a time in the past that has finished.

'Before I got married, I had never been to Canada.'

Notice that the adverb (never, always, just, only, etc.) comes between had and the past participle.

Link: Irregular verb list
Link: How to use' have' and 'has

Now complete the sentences using the past perfect:

• She did not know the time because she (forgot) her watch.
• Manchester United (score) twice before Liverpool did.
• Tom did not go to the concert because he (see) the band many times before.
• I understood the article after I (read) it twice.
• After Kumar (buy) a new sofa, his mother offered to buy him one.
• Ariana (had) that type of dress for a while before they came fashionable.
• Thomas (put) down the phone just as you walked in.