What's the difference between 后 and 以后?

Question

I was taught that so as to say "after" in Chinese one should use 以后, like this:

吃饭以后

Today I came across a bare 后, like this:

15分钟后

What's the difference between these two words?

Answer 1

后= after

Examples:

吃饭后 = after eating dinner

15分钟后 = after 15 minutes

以后 = after

以后 (after) emphasizes on 'time after a specific event or action'

Examples:

吃过那顿晚饭以后 = after eating that dinner (a specific dinner is emphasized)

15分钟的沉默以后 = after 15 minutes of silence (the event- 15 minutes of silence is emphasized)

以后 = 此后 (hereafter); 其后 (thereafter) ; 以来(since)

Example: 自二战以后 = 自二战以来 = since WWII

以后 = 今后 (from now on)

Example:

我以后不再和你说话 = I won't talk to you from now on

Answer 2

I think the main difference is, 后 can also mean behind, the back, the last, whereas 以后 does not take such a meaning.

后面： behind
*以后面 X （not a word)

房后有个车库。 Behind the house is a garage.
我坐在后排。 I'm sitting in the last (the behind) row.

后, 以后 both can mean after, afterwards, later.

后 can also mean children, as in 后生，'those born after' 九零后 'the children of the 90s'

And of course, 'the woman behind the king' i.e. the queen: 帝王的妻子：皇后，太后

Answer 3

后(after,behind), a single character, connotes time or position according to the context. When it refers to time sequencing, it's the same as 以后. It's like in your examples: 吃饭（以）后, 15分钟(以)后. But in 身后，房后 and etc, you can not replace 后 with 以后， because it stands for the location.

以后(after) only connotes position.