Grammar

PRESENT PERFECT 

The present perfect is used for three reasons: 
1. To describe an action that goes from the past up to the present moment. 
2. To describe an action that occurred sometime in the past but the exact time is unknown. 
3. To describe an action that was repeated many times in the past. 

Contractions are usually used with this tense: I've lived, you've lived, he's lived, she's lived, etc.

The formula is:

S + HAVE/HAS + V past participle + C 

1. To describe an action that goes from the past up to the present moment. 
With Non-Continuous Verbs and non-continuous uses of Mixed Verbs, we use the Present Perfect to show that something started in the past and has continued up until now. "For five minutes," "for two weeks," and "since Tuesday" are all durations which can be used with the Present Perfect.

Examples:

· I have had a cold for two weeks.

· She has been in England for six months.

· Mary has loved chocolate since she was a little girl.

Although the above use of Present Perfect is normally limited to Non-Continuous Verbs and non-continuous uses of Mixed Verbs, the words "live," "work," "teach," and "study" are sometimes used in this way even though they are NOT Non-Continuous Verbs.

2. To describe an action that occurred sometime in the past but the exact time is unknown. 
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.

3. To describe an action that was repeated many times in the past. 
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.

· I have had four quizzes and five tests so far this semester.

· We have had many major problems while working on this project.

· She has talked to several specialists about her problem, but nobody knows why she is sick.

ALREADY 

Already means that something happened earlier than we expected. With Present Perfect already usually goes after have or has and before the main verb.

Examples

- We've already had our breakfast.

- When are you going to do your homework?
- But I've already done it!

- Do you want a cup of coffee?
- No, thanks. I've already had one."

JUST 

Use just to express actions that have happened RECENTLY. Just goes after the auxiliary have/has and before the past participle

- Maria has just finished her homework. (not so long ago)
- They have just come from the supermarket.
- It has just started raining.

YET 

We use yet for negative sentences and questions. It is used for actions that you think has happened. It is used to express expectations. (es traducido como ya o aún)

- Have you eaten at "Friday's" yet? (you expect that the other person has already been to "Friday's")
- I haven't been to Europe yet. (But I expect to go there)
- Has Matilda been to New York yet?
- Have you studied for the test yet?
- Juan hasn't proposed to Jenny yet.