I wish and if only

You can use "if only" in place of a Past Wish when you want to express strong regrets. Let's see some examples:

• If only I hadn't drunk a lot last night, the hangover is killing me.
• If only you had been there, you had seen the eclipse.
• If only she had come, she is always the soul of the parties.

Let's see a video:

How to express wishes in English (past, present and future)

Differences between Wishes an Past Wishes sentences

Explaining Past Wishes

Past Wish sentences are different to Wish sentences, because in Wish sentences we are talking about present situations that we desire they occur, moreover, Past Wish sentences refer to past situations that did not occur, they can express:

• Desires to change something that happened in the past. 
• Regrets or dissatisfactions about a past situation.

Examples Past Wishes

Let’s see some examples:

1. I wish I had got that job.
2. I wish you hadn't had that accident, I feel guilty.
3. She wishes she had worn at the party that beautiful dress she saw in the store.
4. We wish we had won the game, the other team defeated us in the last three minutes.
5. He wishes he could have bought the car when he had the opportunity.
6. I wish we could have sung karaoke together in John's birthday.

We can make the same sentences with the following contractions, the meaning will be the same and these are grammatically correct, let’s see:

1. I wish I'd got that job.
2. I wish you'd had that accident, I feel guilty.
3. She wishes she'd worn at the party that beautiful dress she saw in the store.
4. We wish we'd won the game, the other team defeated us in the last three minutes.
5. He wishes he could've bought the car when he had the opportunity.
6. I wish we could've sung karaoke together in John's birthday.

Past Wishes

Grammatically, the Past Wishes have two important elements: the WISH Clause and the THAT Clause. 

WISH Clause + THAT Clause = Past Wishes.

The WISH Clause is formed by: A Noun and wish.

A Noun + wish = WISH Clause

The THAT Clause is formed by: That, a Noun and a sentence in Past Perfect or Could have and Past Participle.

NOTE: the word "that" is often omitted, because it is implied in the general sentence.  

(That) + A Noun + Past Perfect = THAT Clause.
(That) + A Noun + Could have + Past Participle = THAT Clause.

Using "Unless" in The Past Unreal Conditional

In negative sentences, we can have two options:

We can negate both, the IF Clause and the MAIN Clause.

Example:  If I hadn't worked a lot, I wouldn't have been able to buy my house. 

Nevertheless, we can use the word "unless" followed the IF Clause in affirmative way, keeping the same meaning of the sentence.

Example: Unless I had worked a lot, I wouldn't have been able to buy my house.

The Past Unreal Conditional explained easily

Why is The Past Unreal Conditional called Unreal?  

It is not called Unreal because it is an unreal situation, it is called Unreal because it is an action that cannot already be done.

Why is it impossible to do this action?

It is not possible to do the action, because the moment when was possible to do it, is over.

Let's see an example:

If I had known, I would have called you.

• In the previous sentence, we can see that the person who talks cannot already call to his/her listener, because he/she didn't get on time some information.

• If the person who talks had got the information, he/she would have called him/her listener. 

Web site to practice The Past Unreal Conditional

If we want to practice the Third Conditional, we can see this page, where we have some examples and we can check the answers out.

http://www.perfect-english-grammar.com/third-conditional-exercise-1.html

All Conditionals in a image

An image where we can see all conditionals, including the Third Conditional