Hello I have question regarding to usage of present perfect and future perfect tenses.
Let's use examples:

1) by the time you have saved up enough money it will already be an old model.
2) by the time he returns everyone will have forgotten him.

First sentence is " by the time present perfect + future simple."
Second sentence is " by the time present simple + future perfect".

I don't understand when should I use structure. " by the time present perfect + future simple." And when " by the time present simple + future perfect".

Question

Hello  I have question regarding to usage of present perfect and future perfect tenses.
Let's use examples:

1) by the time you have saved up enough money it will already be an old model.
2) by the time he returns everyone will have forgotten him.

First sentence is " by the time present perfect + future simple."
Second sentence is " by the time present simple + future perfect".

I don't understand when should I use structure.     " by the time present perfect + future simple." And  when " by the time present simple + future perfect".

Answer

I find this quite hard to explain but I'll try. In the first sentence, the object will be old at that exact moment in the future we're talking about. So if you imagine we're actually at the time when the person has saved up enough money. The model will be old in the present at that time. With the second example, imagine we're at the point when he has returned. The moment that they have forgotten about him will be in the past at that time.

I think that most native speakers don't really understand the rules and it just comes naturally over time. I hope this helps.