Learn English – Why is the present continuous tense used in “I have not found the ring yet”

Regarding finished past events, we use the

present perfect simple to relate events that occurred in a time period that has not yet finished

and the

simple past for events that took place in a time period that has ended.

American English generally allows for the use of either tense in these situations, while British English grammar insists on using the two tenses to distinguish them.

The arrest took place in the time period of "early this morning", which is apparently assumed to be over at the time of the utterance. So the relevant fact is not that the arrest is finished, but that the early morning is finished.

"This is the third time she's been arrested this year", on the other hand, occurs in an unfinished time period (if this year is not yet over), and so allows for (British: requires) the present perfect.

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:

John hasn't studied very well this term.

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:

John hasn't studied very well this term, so now he's at risk of failing the exam.

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:

John hasn't been studying very well this term.

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:

John hasn't been studying very well this term, so the teacher gave him an extra assignment.

We could also have something like this:

John hasn't been studying very well this term, but he could still catch up.

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).