When a sentence is too long, can I break the sentence by writing a comma before a conjuntion?
Example of sentence:
I am good at explaining ideas, so I explained those ideas to them
When there is too much work, I will ask my friends to share them, or ask my boss to extend the deadline.
They are not interested in music, because music is too hard.
I live in Japan, and I am good at music.
Put the comma before the conjunction if both parts (before and after the conjunction) are independent clauses.
Your second example is therefore incorrect, because you have separated the subject from the verb. However, in real life, people do do this in their writing to show where a natural pause would be. It doesn't make it correct, but it's common. Better advice would be that if you feel the sentence is too long, break it into two sentences.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.