"For how long" vs "How long"!?

[Memento]

Hi everyone :D

I've been wondering about this . . .

What's the main difference between "for how long" & "How long" and their usages .

by searching I found that you could use both with some sentences but you could

only use one of them with others .

Thx for your Help

note : is "others" correct referring to "other sentences"

[Cerberus™]

Welcome!

Others is used correctly.

I have never thought about this difference, but here's my hypothesis.

How long can be used in any situation, including the fewer situations where for how long can be used.

1. How long is the Great Wall?
2. How long / For how long have you been sitting here?
3. How long ago was it that the Mongols last attacked China?

For how long is more particular. I think it comes from "for how long a time". It must always express a period of time: not some other sense of "long" as in 1. It can never be a modifier of another adverb, like in 3 "how long ago": it must always stand on its own as an adverbial constituent.

[Memento]

Hi !

Thx for the explanation . . .

But I'm not sure I got it 100% :(

When we say "For how long do we cook this chicken" or "For how long do we elect our president' , is it ok to take out "for" her

by the way the problem I'm having is with "time usage" not the length :D

I appreciate your fast reply :)

[Cerberus™]

Ah, now I see the problem. I suppose my explanation was too general.
The word "for" in the cooking sentence can be left out: "for how long" says something about the length of time that the cooking takes place in, and can as such be replaced with "how long". But in the "elect" sentence, "for how long" does not say something about the length of time that the electing takes place in: it says something about the consequence or purpose of the electing. Perhaps you could say it stands for "for how long a term". You might however use "how long" here colloquially.
I think the only reason why "for how long" is possible and "how long" is not, in this sentence, is that "for" expresses something more than merely "during"; it normally expresses only "during", in which case "for how long" can always (?) be replaced with "how long".

[Memento]

That's what I was able to Grasp :

in most sentences we could use "how long" generally , but if it's related to a specific period of time we use "for how long'' , though using "how long" here won't so much wrong , it depends on how we mean by the sentence & how the listener understands

.