How do you choose between the Present Perfect Continuous and Past Perfect Continuous?

 

The Present Perfect Continuous is used to talk about an action in progress that was taking place until the present moment.

 

I’m really tired. I have been carrying those bags for half an hour.

 

The Past Perfect Continuous is used to talk about an ongoing action that was taking place until another specified moment in the past.

 

I had been carrying those bags for half an hour when I met Peter and he helped me.

 

Consider the following examples:

 

Present Perfect Continuous	Past Perfect Continuous
I‘m really thirsty. I have been cycling for three hours.	I was really thirsty. I had been cycling for three hours.
They have been playing all day long and now they are tired.	They had been playing all day long before they got tired and went home.
Mary haven’t been doing well at school last month and the results of her tests are poor.	Mary hadn’t been doing well at school last month, so she failed her final tests.