Unreal uses of past tenses

We use it to talk about things we would like to be different in the present / future (but which are impossible or unlikely).

It's a difficult problem. I wish I knew the answer to it!

We talk about things that happened / didn’t happen in the past and that we now regret.

I wish I hadn’t spoken to Jane like that — you know how sensitive she is.

Works exactly like 'wish', but it's more emphatic on the desire:

If only I knew the answer!

If only you hadn’t forgotten the map, we'd be there by now.

We use this structure to express a personal preference:

I'd rather not talk about it.

I'd rather go on my own - thanks!

Here we express a preference over another peson. It's important to know that the subjects don't macth:

I'd rather you stayed at home.

We'd rather she didn't say anything about it.

We use it to indicate that an action has to be done by this moment, without specifying the subject:

It's time to go home!

It's time to start the class.

Works similarly, but here we indicate the subject.

It's time we left!

It's high time she went home!