后 hòu

These began life as two completely unrelated characters. 后 meant 'queen' — the 皇后 in old court dramas. 後 meant 'after' or 'behind'. When mainland China simplified its writing, 後 got folded into 后, so one shape now does both jobs. Taiwan, Hong Kong and Macau never merged them: 以後 is 'after', 皇后 is still 'empress'. If you learn simplified characters, write 后 everywhere and you are always right. You only meet 後 when reading traditional text.

以后 means 'from that point on' and is happy pointing at the future: 以后我要学中文. 后来 only works inside a past story, as the narrator saying 'later on': 后来他搬到北京了. You can never use 后来 about something still ahead of you. 然后 is a plain sequence connector, 'and then', chaining steps in order: 我先吃饭，然后去上班. Quick rule: 以后 = from now on, 后来 = later in that story, 然后 = next step.

Chunking 后 onto a time phrase is completely normal: 三天后, 两年后, 下班后. Adding 以 or 之 gives you 三天以后 and 三天之后, which sound a touch smoother in relaxed conversation. Neither version is wrong, so stop agonizing over it. The real mistake is leaving 后 alone at the front of a clause: 后我回家了 sounds broken. Say 后来我回家了 or 然后我回家了.

Both mean 'behind' or 'the back part', and neither is wrong. 后面 is the standard form you see in textbooks, news and most of China. 后边 turns up in relaxed northern speech and usually points at a physical spot, like a seat or a building. When in doubt, pick 后面 — it never sounds off. The same pattern repeats with 前面/前边 and 里面/里边, and 后头 is the even more colloquial Beijing version.