A2
前に and 後で — before and after
How Japanese orders events with 前に, 後で and てから, and why 前に always takes a non-past verb regardless of the sentence's tense.
Both attach to a clause and place it relative to another. The catch is that each demands a particular verb form, and neither choice has anything to do with tense.
前に takes the dictionary form
寝る前に、歯を磨きます。
Neru mae ni, ha o migakimasu.
I brush my teeth before bed.
日本に来る前に、日本語を勉強しました。
Nihon ni kuru mae ni, nihongo o benkyō shimashita.
I studied Japanese before I came to Japan.
The second sentence is entirely in the past, and 来る is still in the dictionary form. From the standpoint of the 前に clause the event has not happened yet.
日本に来た前に…
日本に来る前に…
前に always takes the non-past form. The tense of the sentence is carried by its final verb — 勉強しました — and nowhere else.
後で takes the past form
食べた後で、散歩します。
Tabeta ato de, sanpo shimasu.
I’ll go for a walk after eating.
Again the sentence is about the future and the verb is past, for the same reason in reverse: within the 後で clause, the eating is done.
With nouns
Both take の: 食事の前に, 授業の後で.
仕事の後で、飲みに行きませんか。
Shigoto no ato de, nomi ni ikimasen ka.
Shall we go for a drink after work?
てから
Also after, but it insists the two events follow each other closely and in that order — a sequence rather than a loose afterwards.
宿題をしてから、遊びます。
Shukudai o shite kara, asobimasu.
I’ll play after I’ve done my homework.
てから implies the first must be finished before the second starts. 後で would allow anything in between.