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Hello , I need help . I'm not an English native speaker and I don't understand some measuresEmotion: smile .

What's the difference in meaning between the use of different tenses in the following sentences :

1.

a.) Come over and see us when our guests leave .
b.) Come over and see us when our guest hav left .

2.

a.) He says he has seen a meteor at some time .
b.) He says he saw the meteor last night .

3.

a.) I was a colleague of hers , working in the same Department , for several years .
b.) I have been a colleague of hers , working in the same Department , for several years .

4.

a.) How far did you get ?
b.) How far have you got ?

5.

a.) She made a fool of herself in public .
b.) She has made a fool of herself in public .

6.

a.) Did anyone go ahead with the project ?
b.) Has anyone gone ahead with the project ?

7.

a.) Did you see the exhibition ?
b.) Have you seen the exhibition ?

Thank you in advance .
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Except for the second pair, where you have reversed the pattern, the first in each case is simple past and the second is present perfect.

Simple past focuses on the event as it occurred at some definite point in time in the past.
When we use simple past we are retelling what happened at that time.

Present perfect focuses on the fact that the even occurred at all, regardless of when it occurred in the past. It is a statement of how things are now, in the present, as a result of this past occurrence at some indefinite point in time in the past.
When we use present perfect we are describing what happened at any time in the past up to the present moment.

Simple past takes place entirely in the past. Present perfect takes place at any time from the past up to the present. Simple past is a recollection of a definite, specific time. Present perfect is a recollection of experiences within an indefinite period of time all the way to the present.

Where X is "now":

......(^^past^^)..............................X......... 'saw the meteor'

......(^^^^present perfect^^^^^^^X)......... 'has seen a meteor'
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Ok , but I want to inquire about what are the differences between those sentences . What for example it means when someone approaches me and asks :

- Did you see the exhibition ?

...and what's the difference when the person asks :

- Have you seen the exhibition ?

and what about the rest of the sentences ? ?
In all cases the differences are analagous. There is little to be gained by examining each in detail. Let's just take the latest examples you cite.

"Did you see the exhibition?" asks if you saw the exhibition, i.e., when the exhibition was held, i.e., when it was possible to go and see it. There is an implication that it may no longer be possible to see it.

.......(exhibition?).........X........... Did you see it at that time?

"Have you seen the exhibition?" asks if you have seen the exhibition, i.e., at any time since it has been possible to see it. There is an implication that it is still possible to see it.
This question asks if, somewhere in the diary of your life experiences, there can now be found the notation: "[DATE] : I saw the exhibition today."
This question asks if you have experienced seeing the exhibition (at some time, no matter when).

.......(e x h i b i t i o n X ? ) ..... Have you seen it yet?

If your own native language does not make this distinction, you will probably not be able to talk yourself into a very satisfying explanation. You will have to "catch on" by imitating native speakers and writers until something "clicks" in your brain!

Emotion: smile