Please, could somebody help me with this. "would" sometimes refers to the past tense, sometimes future. So, I am confused: The original sentence: He would gain considerable advantage from staying in that job.
1) in the above sentence, does "would" refer to past tense, or future?

See a translation

In this example, it's talking about the future. It is saying that if the person stays in the job now, he will likely gain a considerable advantage going forward (in the future).

I believe the technical term for this grammatical structure is "conditional mood"
https://en.wikipedia.org/wiki/Conditional_mood

If the person wants to talk about the imaginary consequences of a (different) past decision, they could instead say "He would have gained"

Example: "He would have gained a considerable advantage from staying in that job"

(meaning if—in the past—he had stayed in that job, he would have gained a considerable advantage as a result).