1. To describe a situation that started in the past and continues in the present.
Ex: "I have studied for two hours"
2. To describe experiences.
Ex: "I have seen elephants in the zoo"
*We can use "ever" to highlight a positive experience, for example:
"My last pijama party was the best day I have ever had!"
3. To describe something that we have done several times in the past and continue in the present.
Ex: He has written two poems and he is working on a new one!
*We can use "ever since" to describe a starting point, but also the starting point of a experience.
"I have played the guitar ever since I was a child".
Ex: "He has just watched "nemo"".
Ex: "Have you ever touched a monkey before?"
Ex: "My mon has gone to Canada for three weeks"
(+)Affirmative: Subject+ have/has+ verb (participle form)+ complement
(-)Negative: Subject+ have not/has not+ verb (participle form)+ complement
(?)Interrogative: Have/Has+ subject+ verb (participle form) + complement+?
Here is a list of verbs that can help you: http://www.easypacelearning.com/design/images/listofirregularverbs.jpg