What is the meaning, difference and usage of before long before then?

Soon is a "very fast, very fast" solution, which is equivalent to very fast. You can use the past tense or the future tense, and you can only be an adverbial alone, not followed by a clause. For example:

Before long, he had to move on.

Soon, he had to move again.

I hope to see you again soon.

I hope to see you soon.

Long before "a long time ago" can be followed by a clause, and before is a conjunction; For example:

Mark learned to read long before going to school.

Mark learned to read long before going to school.

This invention was widely studied long before it was put into use.

The invention was extensively studied before it was put into use.

When before is an adverb, you can also take nothing. For example:

He told me that he had seen the film a long time ago.

He told me that he had seen the film.