**Not a teacher**
It's the subjunctive tense. Basically, you use it whenever you're talking about hypothetical events or events that are unknown/uncertain.
"I will go" is a definite statement (you will go no matter what)
"I would go" is a statement with uncertainties (I would go if i had the money, or I would go, but....)
It is the same tense used for "would" "should" "might" "hope" etc.
Conditionals also get it because they are hypothetical, and therefore uncertain (e.g. If i were king, the people would love me)
There is something like the 'future in past'. It is called the Future perfect. It describes something that will be a past action at some point in the future. "When the sun dies, humans will have been long gone" The future event is the The sun dying, but when that happens, the death of humans will be a past event.
"By the time i graduate, I will have spent 20,000$". The person hasn't spent 20,000$, but he will have at a definite point in the future. Because it is a definite time, it is not subjunctive.
Compare it with "If i were to graduate, I would have spent 20,000 dollars" Which is subjunctive