Examples:

1. I wish I knew = I don't know.
2. I wish you had time = You don't have time.
3. I wish the teacher did something to solve the problem = The teacher doesn't do anything to solve the problem
4. I wish I didn't have to work hard = I have to work hard
5. If only there weren't many people in this place = There are a lot of people in this place

Examples:
1. If only I hadn't failed the Chemistry test = I failed the Chemistry test.
2. I wish I had known that Omar was ill = I didn't know that Omar was ill.
3. I feel sick. If only I hadn't eaten so much cake yesterday = I feel sick. I ate so much cake yesterday.
4. I wish I had applied for that job = I didn't apply for that job.

Examples:
1. I wish I could ride a horse.
2. I wish he would visit me next week.
3. If only I could visit Paris next year.