are these sentences correct?

Hello,

Are these sentences correct and is there any difference between them?

If i went abroad,i would stay as long as i WANT.
If i went abroad,i would stay as long as i WANTED.

If he helped them would you say he IS a good person?
If he helped them would you say he WAS a good person?

If they knew what i THOUGHT of them they wouldn`t invite me to parties anymore.
If they knew what i THINK of them they wouldn`t invite me to parties anymore.

If he knew what his son IS DOING he would never trust him again.
If he knew what his son WAS DOING ...
If he knew what his son HAS BEEN DOING....
If he knew what his son HAD BEEN DOING...

He WAS an engineer for 20 years
He HAD BEEN an engineer for 20 years.

He decided to leave her because he HAD FOUND OUT that she was in love with his friend.
He decided to leave her because he FOUND OUT that she was in love with his friend.

Thank you in advance.