For

For is either a preposition or a conjunction. When it acts as a preposition, it has two main uses: "length of time" and "purpose". In writing, we also use for as a conjunction meaning "because".

1. For (preposition) meaning "length of time"

Let's see some examples below:
for a second
for two hours
for weeks
for ever

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(1) I've been studying English for three years, but I've lived in England only for a short time.

(2) Could you wait here (for) a minute, please?

Note: We omit for in front of all.
Examples: all day, all night, all my life, all the time
They waited here all night. (Not: for all night)

1.1. For with Perfect Tenses

1.1.1. Present Perfect Simple and Progressive

We use for with the Present Perfect Simple (or Perfect Progressive) when referring to a period of time which began in the past and continue up to the moment of speaking (or the moment we are thinking of).

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This car has belonged to my father for years. (= He bought it in 2007 and he still owns it now.)

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The new political party has been developing its' policy for only a year. (= It began developing it last year and is still developing it.)

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1.1.2. Past Perfect

We also use for with the Past Perfect, in talking about a period in the past which lasted up to a specific point in the past.

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Dane had been divorced for 2 years when he met Monica.

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1.2. For with Past Simple

We also use for with the Past Simple, in talking about a completed period of time.

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The course lasted for three months. It finished last week.

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1.3. For with other verb forms

We can use for with any other verb forms to describe a period of time in the future.

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The President is likely to stay in hospital for a month.

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Note: Contrast for with since.
They've been trying to find their lost son for 18 years / since 2006. (It is now 2024.)

2. For (preposition) meaning "purpose"

Examples:
These pills are for headaches .
What did you do that for? (= Why did you do that?)
John is working for Pepsi company.

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Notes:

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(1) There are other meanings of for. For example, for means the opposite of against.
Did you vote for the President or against him?
Also, for sometimes has the meaning of motion: "to go to".
The children left home for school at 6.30 a.m.
For also has the meaning of "intended receiver".
Sam gave his wife a present for the children.

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(2) For also introduces a subject in "infinitive clauses.
The best plan is for Daniel to lend you his car.

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3. For (conjunction meaning "because"

In written English for is sometimes a conjunction meaning the same as "because". We can use for instead of because only when for is:
(a) in the middle of the sentence (not at the beginning), and
(b) before a clause.

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The managers decided to interview everyone, for they all had different opinions. (written only)

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I hope this article helps. If you have any question, please leave a comment in the comment section.