Theory:

The present perfect is a grammatical combination of the present tense and perfect aspect that is used to express a past event that has present consequences
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We form the Present Perfect with
have / has  + the past participle form of the verb (3rd form of the verb).
S +  have / has + V3 + O    
 
S - subject: I, we, you, they, he, she, it, Mary, John ...
V3 - The Past Participle Of The Verb (The Verb In The Third Form): written, eaten, worked, ...
O - object 
Example:
I have travelled a lot.
He has travelled to Europe.
She has lost her pen.
They have studied for the exam.
You have made a beautiful painting.
I have finished my all exams.
They have made a plan for going out.
 
The regular past participles end in -ed.
Example:
work - worked
listen - listened
finish - finished
play - played
snow - snowed
rain - rained
travel - travelled
study - studied
Important!

With irregular verbs, however, the past forms can be same or different. For example:
Example:
sing - sung
sink - sunk
sit - sat
sleep - slept
speak - spoken
spend - spent
write - written
lose - lost
Important!
  
Signal Words: 'already', 'ever', 'just', 'never', 'not yet', 'so far', 'till now', 'up to now'.
 
Ever and neverBefore the main verb (past participle).
AlreadyBefore the main verb (past participle) or at the end of the sentence.
YetPlaced at the end of the sentence.