PRESENT PERFECT
The present perfect is used for sentences that do not satisfy the two main conditions for the past simple: We cannot identify exactly when an action happened in the past and/or if it is finished.

When the sentence contains vague references to time “many times, just, lots of times, ever,” we often cannot point to the exact moment something happened. Therefore, we cannot use the past simple.

We use the present perfect for situations that started in the past and are continuing into the present and beyond, and, therefore, are not finished. Again, in these cases, we cannot use the past simple.

– Formed with the auxiliary “have/has” + past participle

*Remember that the past participle is sometimes different than the past simple form, as in shake, shook, shaken. The verb shake becomes shook in the past simple and shaken is the past participle.

Note that it is called present perfect for a reason. It emphasizes the effect past events have on the present.

I graduated from the University of Parma. – emphasis on finished, past effect
I have graduated from the University of Parma – emphasis on present effect of past action

Have you called the supplier? – Question regarding an action at an indefinite past time.

No, I haven’t called the supplier yet. – Negative statement regarding an action at an indefinite past time.

Yes, I have called the supplier three times. – Multiple actions at indefinite past times.

Ilaria has worked at Chiesi since 2003. – Incomplete action that began in the past. It connects to the present moment.

Antonio has just finished the report. – Completed action that connects to the present.

Signal words for present perfect give you a clue that you should not use the past simple by mistake:

for
since
just
still
yet
already
ever
never

PRESENT PERFECT CONTINUOUS

– Formed with auxillary “have/has” + been + present participle (ending in -ing)

The continuous perfect forms are actually easier to use because you just have to remember have/has been and then add -ing to any verb that comes to mind.

The present perfect continuous puts emphasis on the process.
We tend to use the present perfect continuous with action verbs like cook, play, read, write, speak, run, swim, jump, eat, and drink.

The present perfect continuous is used for things that you have been doing recently/lately.

I have been studying food science since high school.
I have not been calling my mother as often as she would like.
Have you been smoking in between classes?
She has been working on her thesis for months.