Why is 'the perfect tense' used in this sentence?

Question

Darwin drew on four kinds of evidence. He pointed out that some emotional expressions appear in similar form in many nonhuman animals. Some emotions also appear in very young children before much opportunity for cultural learning has occurred. If I were to write this kind of thing, I would do like this about the verb 'occur'."Some emotions also appear in very young children before much opportunity for cultural learning occurs."I don't understand why the present perfect tense' is used.Thank you in advance.

AlpheccaStars · Accepted Answer

anonymousI don't understand why the present perfect tense' is used.

It is difficult to explain, but I will do my best.

Present perfect is necessary in the quoted sentence because it refers to a span of time, and the amount of cultural learning that could possibly have occurred (or had the opportunity to do so) within that time period in a young child's life.

The simple present can be used in this sentence, which refers to a single point in time. It has a different meaning, though.

Some emotions also appear in very young children before any cultural learning occurs.

CalifJim · Answer

After subordinators of time (when, before, after, until, as soon as, ...), the non-perfect and the perfect tenses tend to be in free variation.

We'll welcome her when she [arrives / has arrived].
A lie can run around the world before the truth [gets / has got] its boots on.
Some problems may develop months or years after your treatment [ends / has ended].
It would be better to wait until the government [reopens / has reopened].

One frequently cited explanation of the difference, or of whatever slight difference there is, is that the present suggests a very small amount of time, if any, between events and the present perfect suggests a somewhat greater amount of time between them. Thus until the government reopens suggests waiting until the very moment that the government reopens and then taking action immediately, while until the government has reopened suggests waiting some reasonable amount of time after the government reopens before taking further action.

Another explanation focuses on a difference in point of view. The present sees the action in the subordinate clause as the future seen from the present moment. The present perfect sees the action in the subordinate clause as the past of the future, the point in time where the action in the subordinate clause is already in the past. It's the difference between seeing an action looking forward from the present or seeing it looking backward from the future. Thus when she arrives sees the arrival as occurring in time as the present moves directly into the future, whereas when she has arrived sees the arrival occurring at a time which has already passed at a future time.

There is also the opinion floating about that the perfect form sounds somehow more correct or at least more formal.

The same principles apply in the past.

The crowds were smaller than those that Queen Elizabeth II attracted when she [visited / had visited] these countries.
News of her friend's death was circulating before anyone [found / had found] the body.
The farmer restarted the tractor after it [ran / had run] out of fuel.
I waited until John [finished / had finished] his coffee.

CJ

Why is 'the perfect tense' used in this sentence?

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