could, should, would

[Solomon_13000]

Could you pass the sugar, please?
You should see a doctor.
Would you go out for a date with me?

When do we exactly use could, should and would?.
(In the context of a present tense, future tense
and past tense).

[Solomon_13000]

Is this a good example of explaning the use of could, should, would in
the context of forming sentences in the past tense?.

You could have visited a doctor.
You would have been a doctor if you studied hard.
You should have seen a doctor.

[Mordant]

"Could" indicates capability or possibility.

Capability:
I could call her, but I'm afraid she wouldn't listen.

It also indicates a request.

Could you pass me the salt please?

"Should" indicates desired, correct or recommended action.

You should become a teacher.
People should always tell the truth.

"Would" implies a condition or indicates habitual past action or a request.

I would buy a house (if I hit the lottery). - Condition
When I was younger, I would read poetry to soothe myself. - Habitual action
Would you please pass me the salt? - Request

This is not a matter of tense, by the way. These are all the past tense forms of other modal verbs: can, shall and will. The thing about it is that they are not at all exclusive to expired possibilities, requests, desires, conditions, etc. You can see that in my sentences.

[Solomon_13000]

You could take a train back home.

Is this possibility taking place in the
present or the future?.

[Mordant]

That depends on the intent. It is not clear.

Compare.

Expired:

I bet you really miss living in Fiction City. It was much closer. You could take a train back home. Now you have to fly across the ocean.

Active:

A: I need cheap transportation fast. I just landed a job interview in my hometown.
B: You could take a train back home.