Conditional : Unreal Past

The past tense is sometimes used in English to refer to an 'unreal' situation. So, although the tense is the past, we are usually talking about the present, e.g. in a Type 2 conditional sentence:
If an elephant and a mouse fell in love, they would have many problems.
Although fell is in the past tense, we are talking about a hypothetical situation that might exist now or at any time, but we are not referring to the past. We call this use the unreal past.

Other situations where this occurs are:

after other words and expressions like 'if' (supposing, if only, what if);
after the verb 'to wish';
after the expression 'I'd rather..'

Expressions like 'if'

The following expressions can be used to introduce hypothetical situations:
- supposing, if only, what if. They are followed by a past tense to indicate that the condition they introduce is unreal:

Supposing an elephant and a mouse fell in love? (= but we know this is unlikely or impossible)
What if we painted the room purple? (= that would be very surprising)
If only I had more money. (= but I haven't).

These expressions can also introduce hypothetical situations in the past and then they are followed by the past perfect.

Examples

If only I hadn't kissed the frog (= I did and it was a mistake because he turned into a horrible prince, but I can't change it now.)
What if the elephant had trodden on the mouse? (She didn't, but we can imagine the result!)
Supposing I had given that man my money! (I didn't, so I've still got my money now.)

The verb to wish

The verb to wish is followed by an 'unreal' past tense when we want to talk about situations in the present that we are not happy about but cannot change:

I wish I had more money (=but I haven't)
She wishes she was beautiful (= but she's not)
We wish we could come to your party (but we can't)

When we want to talk about situations in the past that we are not happy about or actions that we regret, we use the verb to wish followed by the past perfect:

I wish I hadn't said that (= but I did)
He wishes he hadn't bought the car (= but he did buy it.)
I wish I had taken that job in New York (= but I didn't, so I'm stuck in Bristol)

NOTE: When we want to talk about situations we are not happy about and where we want someone else to change them, we use to wish followed by would + infinitive:

I wish he would stop smoking. (= I don't like it, I want him to change it)
I wish you would go away. (= I don't want you here, I want you to take some action)
I wish you wouldn't squeeze the toothpaste from the middle! (= I want you to change your habits.)

I'd rather and it's time...

These two expressions are also followed by an unreal past. The verb is in the past tense, but the situation is in the present.
When we want to talk about a course of action we would prefer someone else to take, we use I'd rather + past tense:

I'd rather you went
He'd rather you called the police
I'd rather you didn't hunt elephants.

NOTE: the stress can be important in these sentences, to show what our preference is:

I'd rather you went = not me,
I'd rather you went = don't stay
He'd rather you called the police = he doesn't want to
He'd rather you called the police = not the ambulance service

Similarly, when we want to say that now is a suitable moment to do something, either for ourselves or for someone else, we use it's time + past tense: