On My Free Time

My free time is very precious because I can have it only when my sons are sleeping.

When they fall asleep, I say to myself "Wow!

Finally I have some precious time!"

Then I start to my English study.

And I write in my diary and post an entry to Lang-8.

* A “diary” is a book, for writing personal thoughts. "Diary" means the whole book. If you mean only one part of the book, you need to use "entry".

If you say "my diary" on Lang-8, you're referring to every entry you ever wrote.

Of course I do other things.

In My Free Time

What do you do in your free time?

My free time is very precious because I can have it only when my sons are sleeping.

Or: My free time is very precious because I can only have it when my sons are sleeping.
You can change the place of "only"

Finally I have a wonderful time!".

Then I start to study English.

And I write my diary and add it to Lang-8.

Or: And I write my diary and contribute to Lang-8.

If you say "contribute" it can mean you post and correct.

Or: And I write my diary and post it on Lang-8.

Of course I do other things too.

My free time is very precious because I can have it only during when my sons are sleeping.

When they falls fall asleep, I say to myself, "Wow!

Finally, I got some wonderful free time!".

Then I start to my English study.

Of course, I will also do something else.

I like having a snack snacks and watching TV.