To make things very clear to you, I’ll type a small dialogue that I heard in 'Downton Abbey between the dowager countess and her eldest granddaughter.
A: How long were you planning to wait, before you told us why you really went to London last week?
B: Since you ask, I haven’t decided. So, if I tell you now, will you promise to keep it yourself.
Another line comes from Mrs. Slocombe in ‘Are You Being Served’
M.S.: He’d have to pull the handle a lot of times before he got my cherries up. Mrs. Slocombe and her assistant Ms. Brahms were talking about slot machines.
Before seems to be used commonly in English in this way. But I would like to know why before is followed by a past tense.
Thank you in advance.