It is used to refer to actions that began in the past and continue to the present.

It is used to refer to actions that happened in the past at an unspecified time, and with actions that began in the past and continue to the present.

It is used to refer to actions in progress at the moment.

I‘m currently studying biology at university.

We‘ve already taken the test.

I‘ve been thinking a lot about the situation recently.

I take guitar lessons on Wednesday nights.

It is used to express past actions that continued up to another point in the past.

It is used to express past events that happened BEFORE other past events.

It is used to express events that started and finished in the past.

It is used to express events that were in progress in the past (often when another one-time event happened)

He was sleeping on the couch when I got home.

Before I lost my job, I had been working on some important projects.

They went to the United Kingdom last summer.

By the time we arrived at the train station, the train had already left.