When should I use the Present Perfect Tense? How do I form the Present Perfect Tense?

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We use the Present Perfect tense to talk about unfinished and finished actions.

Unfinished Actions- Habits or states.

Unfinished states or habits are states or habits that started in the past and continue in the present.  We use the present perfect tense to express 'how long'  we have had this habit or  have been in this state. We use either 'since' or 'for' to express 'how long.'

For example:

 We have known each other for 5 years. 
I have known him since January. 


Unfinished Time -Period of time still continuing. 

We use 'this month', 'this week', 'this year' and 'today' with the present perfect to express something that has started and is still continuing. 

For example: 

I haven't seen her this month.
He has cried twice today.
They have visited Colombia 3 times this year.
I have eaten pizza two times this week. 

Finished Actions- Life experience.

When we want to express events or actions that happened sometime during a person's life, we can use the present perfect tense.  When we do,  we don't say when the event or action happened.  The person we are talking about needs to be alive now.

For example:

I have visited Jamaica.
She has taken the test.
We have been to Paris.
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Finished Action- With a result in the present (focus on result).

When we want to express something that happened in the recent past, but  is still true or important now we  use the past simple or present perfect. 

For example:

I've missed the bus. (so I can't go  to work).
They've cancelled our flight. (so she can't travel today).


Finished Action- Something that recently happened

 When we want to introduce news  of something that recently happened we can use 'just', 'yet' or 'already' with the present perfect tense. We can do this even if there is no clear result in the present.  

For example:

I've already eaten. 
I've just heard her speech.


How do I form the Present Perfect Tense?

To form the Present Perfect tense we use has or have and the past participle of the verb in the sentence. 

For example:

We have known each other for 5 years. 

In the sentence above, 'we' is the subject so we use 'have' and NOT 'has.' The word 'known' is the past participle.
 
In this video I explain how you can form the present perfect tense.