Present Perfect

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.
• I think I have met him once before.
• There have been many earthquakes in California.
• People have traveled to the Moon.
• People have not traveled to Mars.
• Have you read the book yet?
• Nobody has ever climbed that mountain.
• A: Has there ever been a war in the United States?
  B: Yes, there has been a war in the United States.

He concept of "unspecified time" can be very confusing to English learners. It is best to associate Present Perfect with the following

Present Simple

Use the Simple Present to express the idea that an action is repeated or usual. The action can be a habit, a hobby, a daily event, a scheduled event or something that often happens. It can also be something a person often forgets or usually does not do.

Examples:

• I play tennis.
• She does not play tennis.
• Does he play tennis?
• The train leaves every morning at 8 AM.
• The train does not leave at 9 AM.
• When does the train usually leave?
• She always forgets her purse.
• He never forgets his wallet.
• Every twelve months, the Earth circles the Sun.
• Does the Sun circle the Earth?