I have a shorter skirt, but my mom wouldn't let me wear it. 為什麼這裡要用would, 我知到would 是will的過去式,我也知道它可以用來表禮貌,但我卻常 不知該何時使用它?

would可以用在表示[單純的未來]，與過去無關。
意思是:將[會]
He would be absent when we are most busy!
＝我們最忙的時候，他總是不在。（習以為常地做...）

"Stop teasing me or I'll tell mama."＝不要作弄我，不然我要告訴媽媽。
"You would!" =我知道你會的。

The door would not open.
=那扇門怎麼也打不開。

p.1607 <新世紀英漢辭典>

You say that someone "would" do something when it is typical of them and you are critical of it.
p.1811 Collins CoBuild English Dictionary for Advanced Learners 3rd edition

would is used to form conditional statements.
eg:-would be impolite if shorter skirt worn.
eg:-would have been impolite if shorter skirt had been worn.

Jeff wants to put on a short skirt? That is awkward indeed!

①為什麼這裡要用would, ⇒（表示推測）大概，會　例. I think he would accept the invitation. 我想他會接受邀請的。
我知到②would 是will的過去式, ⇒ 多重(9項?)釋義之一。
我也知道它可以③用來表禮貌, ⇒ 多重(9項?)釋義之一(二)。
但我卻常 ④不知該何時使用它? ⇒ 查閱辭典，學習(磨劍練功夫)。