Verona: 1.By 1999 he had lived there for ten years - the Past Perfect
2. By the age of 16, she learned German - the Past Simple
Frankly speaking, I see no difference between the sentences in terms of grammar. Both of them imply that the action/state preceded a particular moment in time (which was in the past), expressed by the preposition 'by'.
Would saying 3. "by the age of 16 she had learned German" make the sentence sound different[STRIKE]ly[/STRIKE]?
5jj: Although some people might say #2, #3 is the version we would expect. #2 could have the implications suggested by keannu (below).
Verona: Talking about the future:
4.I'll do it by Wednesday
5. I'll have done it by Wednesday.
Is the choice between Simple/Perfect Simple just a matter of taste here?
5jj: There is not a great deal of difference. #5 makes it crystal clear that the 'doing' will be completed,
keannu:.
By the age of 16, she had learned German.
From native teachers, I guess "by" needs an action completed at the time that comes after "by" as "by" has the nuance of the moment of completion, so if you just say learn, it would mean "you start to something" possibly at the last moment that doesn't go with expected completion .
5jj: This is one possibility. The other is that the speaker meant to express completion, but omitted to use the past perfect.
Keannu: But I don't see if I can change the following example like this. Please,teachers help me!
A: Can I borrow your hammer for a moment?
B: No, I'm using it. Please wait until I (finish, have finished) No real difference.
=> Please come back by the time I have finished(not finish!!!) Correct. See my post #5. (Although I have to say that this sounds a little unnatural. It assumes that the listener knows when the speaker will finish.)
The real problem for learners is that native speakers are not always precise about using the past perfect in the way grammar books suggest it is used.