Rule: The present perfect expresses a state or event that occurred at some indefinite time during a period extending from some past time up to the moment of speaking. The past tense expresses something that happened at a definite time in the past.
Examples of errors:
In (10)-(26) there is a past time adverbial expressed or implied that requires the simple past rather than the present perfect. Part of the problem for the German learner is not so much distinguishing between definite and indefinite past time reference, but simply the tendency to equate the English pre,sent perfect construction with the German Zusammengesetzte Vergangenheit:
I saw the Cosmos in 1976.
Ich habe in 1976 die Cosmos gesehen.
"Definite" in this context, as opposed to "indefinite", .means anything more specific than 'at some time in the past up to now'. The clause when I was a student, for example, is not very precise, but it is definite enough to require the simple past:
When I was a student I played cards a lot.
*When I was a student I've played cards a lot.
Even an expression like for six weeks (cf. (18)), which specifies the length of time but nothing about the relation of the period to the moment of speaking, makes the reference definite:
Have you (ever) been to England? (indefinite reference)
Yes, I was there for six weeks (definite reference)
(Have been to means 'have gone (traveled) to', and cannot take an adverbial of time duration like for six months (cf. (25)).
Sometimes it is the addition not of a time adverbial, but of some phrase which gives more information about the event, which has the effect of making the past time reference definite. For example, in (17) the indefinite reference to having received a gift becomes definite when the information about the price is added. It is quite common for the present perfect to occur first, when a new subject is introduced into the discourse, and then for the simple past to be used in the ensuing remarks. Here is another example. Just returning from the supermarket one might say (compare (24)):
I've bought (or I bought) a lot of stuff today. I bought that salami you like, some fresh broad ...
A further restriction is that the indefinite past time period to which the present perfect refers must, as with all uses of the present perfect, extend up to the moment of speaking. (16) would be acceptable if Daniel were still at school, or if he had just left school, but neither is the case here. Likewise, (21) occurs in an essay written in class after the Christmas holidays, so the time period referred to does not extend up to the present. Once the time reference is established as definite past, it is not possible to shift to the present perfect (time up to the present), unless an entirely new subject is introduced which would justify the change (cf. (12), (19), (20), (22)).
What is sometimes called the resultative perfect (cf. (3)-(9)) emphasizes the present consequence or result of an indefinite past event. This is due to the fact that certain verbs (break, cut become, obtain, arrive, etc.) express events or actions that involve a transition from one state or condition to another (cf. 2.3.2). Because this idea of transition is so prominent a part of the meaning of these verbs, their use in the present perfect focuses attention not so much on the fact that the past event occurred, but on the new and present state which has resulted from the event. For example, the sentence
I've seen a lot of football games.
makes a statement about the indefinite past, which one might paraphrase as 'I am now in the condition of having seen a lot of football games'. But since the contrast between the states obtaining before and after having seen football games is not obvious or dramatic, our attention is not focused on the present result (if there can be said to be a result) of the past events. By contrast, sentences like
He's broken his leg (i.e. his leg is broken).
I've forgotten your name (i.e. I can't remember it).
The sun has gone down (i.e. it's dark).
He's changed a lot (i.e. he's different).
have almost the effect of a present tense, since the consequence of the past event is dramatic, and more important than the event itself. The meaning of this resultative perfect is basically that described in Rule 2.4.2, however, since it still involves indefinite past time reference, and precludes the occurrence of a definite time adverbial:
*He's broken his leg last week.
*I've forgotten your name yesterday.
(1) and (2) are examples of indefinite past time reference without resultative meaning. (1) occurs at the end of a letter, and the time frame is 'from the time I started writing (or you started reading) up to now (2) is a negative sentence, semantically speaking, since it refers to the non-occurrence of the predication (having to face problems) during the period encompassing the entire past up to the present.