When I am rich, I will buy a car
When he is rich, he will buy a car

If I have time, I will come here
If he has time, he will come here

As soon as I have done it, I will let you know
As soon as he has done it, he will let me know

Once I'm free, I will take a trip abroad
Once he is free, he will take a trip abroad

Unless I come back tomorrow, he will leave
Unless he comes back tomorrow, I will leave

Does these sentences correct grammatically?

Question

When I am rich, I will buy a car
When he is rich, he will buy a car

If I have time, I will come here
If he has time, he will come here

As soon as I have done it, I will let you know
As soon as he has done it, he will let me know

Once I'm free, I will take a trip abroad
Once he is free, he will take a trip abroad

Unless I come back tomorrow, he will leave
Unless he comes back tomorrow, I will leave

Does these sentences correct grammatically?

Answer

They are all correct, except:

"Are these sentences correct?"
  🙂

Answer

My English level is not very good, and I can't read or understand it. I started learning from the foundation of 0. What is the purpose of learning English? go abroad

Answer

Your use of contractions is inconsistent in your examples. In speech and informal writing, it’s unnatural not to say
I’m, I’ll, he’s, he’ll etc.

In the third pair, I’ve and he’s

In the second pair, where ‘have’ has a different meaning, you shouldn’t use a contraction for it.

Answer

All are perfect.

With only a tiny change in meaning, you can also say:
"As soon as I do it, I will let you know."

The second half of each sentence can be changed to present tense by using participles. There is a tiny change in meaning. For example:
"When I am rich, I am buying a car" or "I am going to buy a car".
The same can be done with all of your sentences.