Past perfect, and how different they are to simple tense

Question

In the evening, the children told their father what they had seen at the zoo.

In the evening, the children told their father what they saw at the zoo.

Now i understand both the sentences are grammatically correct. What i want to know is, what's the point of using the past perfect tense there? I mean, it does make it more clear that the kids went to the zoo first, and told their father what they had seen there second. Is there any other reason why we should use the past perfect there?

And

When Jane had seen the elephants, she wanted to see the giraffes. (second action happened after the first action had been completed)

Could we not mean the same by saying "After jane saw the elephants, she wanted to see the giraffes?

Answer 1

The past perfect (or pluperfect) establishes the order of events in sentences like these. The past simple does not. Consequently, where the order is important, it may seem unnatural or deliberately vague to use the past simple.

In the above sentences, there are enough clues to determine the order of events.

But consider:

The children described to their father what they saw through the bars.

It seems more likely that the children are narrating to their father nearby, rather than telling him later. If they are recounting their visit to the zoo it would be clearer - and more natural - to say:

The children described to their father what they had seen through the bars.

You ask about:

When Jane had seen the elephants, she wanted to see the giraffes.

And as you say, this is essentially the same as:

After Jane saw the elephants, she wanted to see the giraffes.

But it is not the same as:

When Jane saw the elephants, she wanted to see the giraffes.

... which implies that as soon as Jane first set eyes on the elephants, she decided she wanted to see giraffes.

Consider also:

I told Jane that the giraffes spent the morning sleeping.

Which did I say?

• "The giraffes spend the morning sleeping."

• "The giraffes have spent the morning sleeping."

The past perfect would make it clear that the second was true.

Answer 2

Unfortunately, as I discussed in this recent answer, these distinctions are much more subtle than rules in grammar books would suggest, although rules in grammar books have their purpose.

In practice there's not a lot of difference between these two. The subtle difference here is, had seen places emphasis on the completed nature of the event, as though they had a bunch of animals on a "to do" checklist and the father wondered which ones they had checked off. Cf What movies have you seen? (a list) or Have you ever seen a hippopotamus? (a life experience, so to speak).