PERFECTIVE OR IMPERFECTIVE?
Now we compare the following sentences:
I haven’t seen Paul this morning.
I didn’t see Paul this morning.
These sentences seem to be identical, only the form of the verb is different. In fact, using different tenses indicates different meanings.
The use of Present Perfect as in the first sentence suggests that the sentence was said in the morning, i.e. in the already mentioned morning.
Past Simple in the second sentence means, that the morning is over, the period is finished, and the sentence could have been said in the afternoon or evening.
Here’s another pair of sentences:
I have been to London.
I went to London in 1993.
We use Present Perfect when we talk about something we did in the unspecified time of our life between the moment of our birth and the moment of our present.
Simple Past, on the other hand, is used when there is a specific time e.g. 1990. yesterday.
A sentence in Simple Past often complements the information contained in a sentence from Present Perfect.
A: I have found my wallet!
B: When did you find it?
A: Ten minutes ago. It was behind those books.
WHEN OR HOW LONG?
Present Perfect often expresses the duration of activity:
I’ve known her for a long time.
We’ve been married for over 8 years.
Past Simple indicates when something happened:
I met her 12 years ago.
We got married in 1995.
It is not acceptable to use Present Perfect with a specific date (e.g. last year, in November, etc.), while using Simple Past with an expression indicating the duration of the action is correct, but completely changes the sense of the sentence. Then the sentence is not about the duration of the activity “from the past; to the present”, but “once in the past”:
I knew her for many years. We were married for over 8 years.
Present Perfect Tense
This tense describes activities that started in the past and continue to the present.
We use it to describe activities that took place in an unspecified time – from some point in the past to the present.
It indicates how long the action lasts.
Past Simple Tense