When do we use it?

We use wish and if only to talk about things that we would like to be different in either the present or the past. If only is usually a bit stronger than wish. 

Present

We can use wish/if only + a past form to talk about a present situation we would like to be different. 

After wish/ if only you get a subject + past simple

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I wish you didn't live so far away.

If only we knew what to do.

He wishes he could afford a holiday.

Past​

We can use wish/if only + a past perfect form to talk about something we would like to change about the past. 

After wish/if only you get a subject + past perfect (had + volt dlw).

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They wish they hadn't eaten so much chocolate. They're feeling very sick now.

If only I'd studied harder when I was at school. 

Expressing annoyance

We can use wish + would(n't) to show that we are annoyed with what someone or something does or doesn't do. We often feel that they are unlikely or unwilling to change.

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I wish you wouldn't borrow my clothes without asking.

I wish it would rain. The garden really needs some water.

She wishes he'd work less. They never spend any time together.