It would be possible to write a short book on how to use this tense. Many students of English find it very difficult to understand. We use THE PRESENT PERFECT TENSE and THE SIMPLE PAST TENSE to describe things which happened in the past. The difference is that we use THE PRESENT PERFECT TENSE when things which happened in the past are connected to what is happening now.
Examples might help:
PRESENT PERFECT: I’ve eaten loads of chocolate (I ate the chocolate in the past but I feel sick now)
PAST SIMPLE: I ate loads of chocolate (This happened when I was younger or maybe yesterday. I can’t feel that chocolate in my stomach now. I don’t feel sick or full)
PRESENT PERFECT: My boss has asked me to do a presentation (Although my boss asked me last week, the presentation is not until tomorrow and I am worried about it now)
PAST SIMPLE: My boss asked me to do a presentation (This happened last week. I did the presentation and now I am not thinking about it)
PRESENT PERFECT: I haven’t had the opportunity to travel much (Happily, my life is continuing. I am not dead! I might have the opportunity to travel in the future. This situation is not finished)
PAST SIMPLE: I didn’t have the opportunity to travel much when I was younger because I was quite poor (This situation is not true now. You are not ‘younger’ and you probably can travel more)
PRESENT PERFECT: Many people say that the summers have become wetter and the winters have become milder as a result of global warming. (We don’t know exactly when this happened, but it definitely started in the past and it affects us now)
PAST SIMPLE: There is some evidence that in the sixteenth century, summers became wetter and winters became milder (This happened and finished in the past. In the seventeenth century, summers and winters were normal)
There are some words connected to time which you usually use with PRESENT PERFECT TENSE:
I’ve RECENTLY/JUST changed jobs (RECENTLY and JUST can be used to describe something which happened a very short time ago)
She’s worked here SINCE she left school (SINCE is used to describe a period from one moment in the past until now)
I’ve done some exercise every day FOR the last six months (FOR is used to describe a period of time which started in the past and continues until now)
DURING/OVER the past twenty years, there has been a marked increase in the number of eighteen-year-olds deciding to take a gap-year before entering university. (DURING and OVER are very similar to FOR. They describe a period of time which started in the past and continues until now. They are sometimes used with longer periods and sometimes used to describe changes.
AN IMPORTANT RULE:
With a word/phrase which describes a finished past time, we don’t use PRESENT PERFECT TENSE. We often use SIMPLE PAST TENSE:
Last week, I saw three old schoolmates.
When I was little, I hated ice-cream.
In 2001, average rainfall fell.
In the sixteenth century, life expectancy was incredibly low.
This is not a complete explanation but it should help. When you are reading or listening to English, and you see or hear a PRESENT PERFECT TENSE, look at this explanation and try to work out why PRESENT PERFECT TENSE has been used.
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