the first use of present perfect is to show an action which began in the past and continue to present. this use is easy
but the second use of present perfect is to show an action finish in the past but is relevant to present. this use is also popular and hard to use. most mistakes we can not realize is in this use.
to understand this use, we need to know simple past. it is used to show anchor an action to a point of time or period in the past. this point or period must be specific. present perfect showing past action dose not show specific point or period in the past. we say the action finished and dont say the point or period in the past . in so doing, we means the past action relevant to present.
concept of relevance to present time is simple. if we dont say the specific point or period of the past, we mean the past action is relevant to present time. this is all about "relevance to present time". dont think too much about "relevance" here. we want to say at present that the action finished in the past. this means the past action is relevant to the past.
we can say that if there is no specific point or period in the past, we use present perfect. correct.
some writings on "relevance to present time" are made in grammar books. but this is simple as I have just explained.
the second use of present perfect is used correctly and naturally by my native speakers . but for non native , it is hard.
understanding this second use helps us understand simple past and present simple. simple past, simple present and present perfect are 3 most basic and hardest tenses in english and is not explained fully in most grammar books.
we should review these 3 tenses to master english tenses. gmat is very hard and basic and certainly will test these tenses frequently on SC section
for simple present.
1, show a condition which exist indefinitely, meaning it exist before, in , and after the moment of speaking
the whether here is nice.
2, show an action which exist at moment of speaking.
I push the door now
3. show an habitual action. action happen frequently before and after the moment of speaking. this use is similar to 1 st use.
so, in fact, there are onlyy 2 uses of simple present.
some use of present are used to talk about past action of advertisement is not basic to know