11 graders: Past Perfect and Past Perfect Continuous

Past Perfect is used to express an action that occurred before another one in the past. For instance, check this 

example:

When I arrived to ckass, the test had already begun. 
               recent action                 oldest action


There are time expressions that are used with this tenses: for, since, just, when, never, until,etc. 


Past Perfect Continuous is similar to Past Perfect, but it gives emphasis to the duration of the action. You might have 1) time expressions that help you to understand that the action is long, or 2) you have the action itself which is long, for example go for a walk, read a book, drive a car, etc.

There are time expressions that appear with this verb tense: for, since, how long

Example:

He had been studying for two hours when his mum called him. 


Now, to help you clarify your ideas watch this video:

Finally, exercises here!