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Present Perfect
FORM
[has/have + past participle]
Examples:
* You have seen that movie many times.
* Have you seen that movie many times?
* You have not seen that movie many times.
Complete List of Present Perfect Forms
USE 1 Unspecified Time Before Now

We use the Present Perfect to say that an action happened at an unspecified time before now. The exact time is not important.You CANNOT use the Present Perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in Japan, at that moment, that day, one day, etc. We CAN use the Present Perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, etc.
Examples:
* I have seen that movie twenty times.* People have traveled to the Moon.
* People have not traveled to Mars.
* Have you read the book yet?
* A: Has there ever been a war in the United States?
B: Yes, there has been a war in the United States.
How Do You Actually Use the Present Perfect?
The concept of "unspecified time" can be very confusing to English learners. It is best to associate Present Perfect with thefollowing topics:
TOPIC 1 Experience
You can use the Present Perfect to describe your experience. It is like saying, "I have the experience of..." You can also use this tense to say that you have never had a certain experience. The Present Perfect is NOT used to describe a specific event.
Examples:
* I have been to France.
This sentence means that you have had the experience of being in France.Maybe you have been there once, or several times.
* I have been to France three times.
You can add the number of times at the end of the sentence.
* I have never been to France.
This sentence means that you have not had the experience of going to France.
* I think I have seen that movie before.
* He has never traveled by train.
* A: Have you ever met him?
B: No,I have not met him.
TOPIC 2 Change Over Time
We often use the Present Perfect to talk about change that has happened over a period of time.
Examples:
* You have grown since the last time I saw you.
* The government has become more interested in arts education.
* Japanese has become one of the most popular courses at the university since the Asian studies program was established.
* MyEnglish has really improved since I moved to Australia.
TOPIC 3 Accomplishments
We often use the Present Perfect to list the accomplishments of individuals and humanity. You cannot mention a specific time.
Examples:
* Man has walked on the Moon.
* Our son has learned how to read.
* Doctors have cured many deadly diseases.
* Scientists have split the atom.
TOPIC 4 An Uncompleted ActionYou Are Expecting
We often use the Present Perfect to say that an action which we expected has not happened. Using the Present Perfect suggests that we are still waiting for the action to happen.
Examples:
* James has not finished his homework yet.
* Susan hasn't mastered Japanese, but she can communicate.
* Bill has still not arrived.
* The rain hasn't stopped.
TOPIC 5 MultipleActions at Different Times
We also use the Present Perfect to talk about several different actions which have occurred in the past at different times. Present Perfect suggests the process is not complete and more actions are possible.
Examples:
* The army has attacked that city five times.
* We have had many major problems while working on this project.

Modal Verbs

Modal verbs arespecial verbs which behave very differently from normal verbs. Here are some important differences:

1. Modal verbs do not take "-s" in the third person.
Examples:

* He can  speak Chinese.
* She should  be here by 9:00.

2. You use "not" to make modal verbs negative, even in Simple Present and Simple Past.

Examples:

* He should not be late.
* They might not...