>"If I had more time, I could have traveled the world" or " If I were you, I would buy a house."
The first should be changed to:
"If I had more time, I could travel the world" to be used in present time.
It also must be changed to:
"If I had had more time, I could have traveled the world" to be used in past time.
> why did they use HAD and WERE? isn't that past tense?
Not in these cases. Here, they represent conditional (see the IF) or subjunctive mood forms. Same words, different meanings, depending on context.