In the book "English Grammar in Use" (Murphy Raymond), I encountered the following example when talking about present perfect tense:
John hasn't studied very well this term.
and explains the use of present perfect simple tense as follows:
We use the present perfect tense when the period is not finished at the time of speaking
I, then, applied this rule to the example above which led me to think that the unfinished period here is this term. The speaker might be saying this fact before the end of the term.
However, the next units compare these two tenses and say:
PP Continuous: We are interested in the action. It does not matter whether something has been finished or not.
PP Simple: This time, the important thing is that something has been finished. We are interested in the result of the action, not in the action itself.
Having read this, I started confusing. The above example used the present perfect simple tense even if we do not know whether John has changed and started studying well for the rest of the term. So, in the example, we do not seem to be interested whether the action has been finished or not (as PP Continuous was said to be used in this case).
Would not it be, then, PP Continuous as
John hasn't been studying very well this term.
Best Answer
In general, the choice of grammatical aspect is, unlike tense, not strictly delineated. You can often choose between two or more aspects to express the same basic idea, because aspects have less to do with denotation, and more to do with connotation. In other words, aspects carry implications and tonality, but they have less effect on the literal meaning of the sentence. As a result, the "correct" aspect is often a matter of opinion and interpretation.
So, let's go through these examples:
This is present perfect simple, as your book says. It carries the connotation that John's poor study habits are either already causing problems for him, or are likely to cause problems in the near future. For example, you might continue the sentence like this:
The emphasis is on the consequences of the action, even if those consequences are not explicitly stated.
Now, let's look at your other example:
This is present perfect continuous. Since it's still a perfect aspect, it still conveys the same emphasis on consequences. However, it also suggests that there is still time for John to change his study habits. It sometimes shows up in cases where the action is interrupted or altered, as in this continuation:
We could also have something like this:
These continuations are (for the most part) also possible with the simple perfect, but they make more logical sense in the context of the continuous perfect.
Having said all that, you generally cannot use the present perfect continuous in cases where the action is far in the past, or otherwise "complete." Similarly, the simple perfect is usually considered less appropriate for a continuous action which is still ongoing (although, as in our examples above, it can sometimes be used with habitual or repeated actions, especially when those actions are unlikely to continue).