First of all, I've been told that you can use the future simple tense when you promise/intend to do something not later than some date, as in:

"I'll do my homework by 9 pm".

You can use the future perfect if you confidently predict something:

"I will have done my homework by 9 pm".

I haven't seen any explanation for this in Grammar books (which is odd), but, nonetheless, it's what I've been told on this forum.

I have been comparing two resources (The first and the second) which seem contradictory:

The first states:

Don't use certain strict expressions denoting time that require certain complicated tenses. For example, "by the time" requires the perfect tenses. If you replace "by the time" with "before"..., you can use the simple tenses...

I will have finished the report by six. I will finish the report before six.

The second:

Sometimes, you can use the future perfect tense and the simple future tense interchangeably. In these two sentences, there is no real difference in meaning because the word before makes the sequence of events clear:

Linda will leave before you get there. Linda will have left before you get there.

But without prepositions such as before or by the time that make the sequence of events clear, you need to use the future perfect to show what happened first.

Here are two sentences I've made that should mean the same and be correct according to the second source:

 A) Mary will leave by the time you get there.

 B) Mary will have left by the time you get there.

To recapitulate, according to the second source "there is no real difference in meaning" in the sentences A and B and I "can use the future perfect tense and the simple future tense interchangeably"

Whereas, according to the first source "by the time" requires the perfect tenses, that is I cannot use the simple future tense in A.

So, which source is correct?

I've come across this:

Sometimes, you can use the future perfect tense and the simple future tense interchangeably. In these two sentences, there is no real difference in meaning because the word before makes the sequence of events clear:

Linda will leave before you get there. Linda will have left before you get there.

But without prepositions such as before or by the time that make the sequence of events clear, you need to use the future perfect to show what happened first.

That means: "Mary will leave by the time you get there." is equal to "Mary will have left by the time you get there."

So, is the source correct?

First of all, I've been told that you can use the future simple tense when you promise/intend to do something not later than some date, as in:

"I'll do my homework by 9 pm".

You can use the future perfect if you confidently predict something:

"I will have done my homework by 9 pm".

I haven't seen any explanation for this in Grammar books (which is odd), but, nonetheless, it's what I've been told on this forum.

I have been comparing two resources (The first and the second) which seem contradictory:

The first states:

Don't use certain strict expressions denoting time that require certain complicated tenses. For example, "by the time" requires the perfect tenses. If you replace "by the time" with "before" or "after", you can use the simple tenses, especially in everyday speech.

I will have finished the report by six. I will finish the report before six.

The second:

Sometimes, you can use the future perfect tense and the simple future tense interchangeably. In these two sentences, there is no real difference in meaning because the word before makes the sequence of events clear:

Linda will leave before you get there. Linda will have left before you get there.

But without prepositions such as before or by the time that make the sequence of events clear, you need to use the future perfect to show what happened first.

Here are two sentences I've made that should mean the same and be correct according to the second source:

A) Mary will leave by the time you get there.

B) Mary will have left by the time you get there.

To recapitulate, according to the second source "there is no real difference in meaning" in the sentences A and B and I "can use the future perfect tense and the simple future tense interchangeably"

Whereas, according to the first source "by the time" requires the perfect tenses, that is I cannot use the simple future tense in A.

So, which source is correct?

First of all, I've been told that you can use the future simple tense when you promise/intend to do something not later than some date, as in:

"I'll do my homework by 9 pm".

You can use the future perfect if you confidently predict something:

"I will have done my homework by 9 pm".

I haven't seen any explanation for this in Grammar books (which is odd), but, nonetheless, it's what I've been told on this forum.

I have been comparing two resources (The first and the second) which seem contradictory:

The first states:

Don't use certain strict expressions denoting time that require certain complicated tenses. For example, "by the time" requires the perfect tenses. If you replace "by the time" with "before"..., you can use the simple tenses...

I will have finished the report by six. I will finish the report before six.

The second:

Sometimes, you can use the future perfect tense and the simple future tense interchangeably. In these two sentences, there is no real difference in meaning because the word before makes the sequence of events clear:

Linda will leave before you get there. Linda will have left before you get there.

But without prepositions such as before or by the time that make the sequence of events clear, you need to use the future perfect to show what happened first.

Here are two sentences I've made that should mean the same and be correct according to the second source:

A) Mary will leave by the time you get there.

B) Mary will have left by the time you get there.

To recapitulate, according to the second source "there is no real difference in meaning" in the sentences A and B and I "can use the future perfect tense and the simple future tense interchangeably"

Whereas, according to the first source "by the time" requires the perfect tenses, that is I cannot use the simple future tense in A.

So, which source is correct?

A) Mary will leave by the time you get there.

B) Mary will have left by the time you get there.

C) Mary had left by the time you got there. (I'm sure this is correct)

D) Mary left by the time you got there. (I surmise this is utterly wrong)

I have been comparing two resources (The first and the second) which seem contradictory:

The first states:

Don't use certain strict expressions denoting time that require certain complicated tenses. For example, "by the time" requires the perfect tenses. If you replace "by the time" with "before" or "after", you can use the simple tenses, especially in everyday speech.

I will have finished the report by six. I will finish the report before six.

The second:

Sometimes, you can use the future perfect tense and the simple future tense interchangeably. In these two sentences, there is no real difference in meaning because the word before makes the sequence of events clear:

Linda will leave before you get there. Linda will have left before you get there.

But without prepositions such as before or by the time that make the sequence of events clear, you need to use the future perfect to show what happened first.

To recapitulate, according to the second resource "there is no real difference in meaning" in the sentences A and B and I "can use the future perfect tense and the simple future tense interchangeably"

Whereas, according to the first resource "by the time" requires the perfect tenses, that is I cannot use the simple future tense in A.

Now, what source is correct? Am I misinterpreting something? Which sentences are correct?

First of all, I've been told that you can use the future simple tense when you promise/intend to do something not later than some date, as in:

"I'll do my homework by 9 pm".

You can use the future perfect if you confidently predict something:

"I will have done my homework by 9 pm".

I haven't seen any explanation for this in Grammar books (which is odd), but, nonetheless, it's what I've been told on this forum.

I have been comparing two resources (The first and the second) which seem contradictory:

The first states:

Don't use certain strict expressions denoting time that require certain complicated tenses. For example, "by the time" requires the perfect tenses. If you replace "by the time" with "before" or "after", you can use the simple tenses, especially in everyday speech.

I will have finished the report by six. I will finish the report before six.

The second:

Sometimes, you can use the future perfect tense and the simple future tense interchangeably. In these two sentences, there is no real difference in meaning because the word before makes the sequence of events clear:

Linda will leave before you get there. Linda will have left before you get there.

But without prepositions such as before or by the time that make the sequence of events clear, you need to use the future perfect to show what happened first.

Here are two sentences I've made that should mean the same and be correct according to the second source:

A) Mary will leave by the time you get there.

B) Mary will have left by the time you get there.

To recapitulate, according to the second source "there is no real difference in meaning" in the sentences A and B and I "can use the future perfect tense and the simple future tense interchangeably"

Whereas, according to the first source "by the time" requires the perfect tenses, that is I cannot use the simple future tense in A.

So, which source is correct?