- Is there a specific time mentioned?
The Simple Past is used when the time is CLEAR:
They met on Sunday.
My birthday was last week.
John started his business after he graduated.
(We know exactly when.)
The Present Perfect is used when the time is NOT SPECIFIC:
They have met already.
I have celebrated my 20th birthday.
John has started his business.
(We don't know exactly when.)
I found the above info here and in some other books.
I concluded that there is a difference between using simple past and present perfect when the exact time is not specified, i.e. using simple past in this case is not considered correct form. The reason I concluded this way is that if they were interchangeably usable in the case of indefinite time in past, it should have been mentioned both tenses are usable. But there is not any such thing here.
Is this conclusion wrong, according to American English?
Best Answer
There's a lot of confusion around this, and a lot of half-true "rules" are taught.
The actual rule for the present perfect is fairly simple. The present perfect is a present tense, designating a present situation, so it cannot be used with a temporal expression which does not include the present, the moment at which the sentence is uttered.
Consequently:
Note, however, that an entirely past temporal expression may be included in a present perfect sentence if it is 'bracketed' with commas or dashes or parentheses; it is then understood as a 'supplement', not integrated into the sentence but added to the sentence as an afterthought.
ADDED:
DamkerngT points out that you conclude that use of simple past with an indefinite temporal expression is incorrect. This is not the case. The simple past can be used with no temporal or with indefinite temporals:
The only sort of temporal which cannot be used with the simple past is one which cannot be interpreted as lying entirely in the past, such as now or currently or tomorrow.