Ytukyg

This grammar article says 以後 must modify some time, action, or event.

However, one of the examples is: 以后再也不来这里吃饭了，又贵又难吃.

1) In the above example, 以後 starts the sentence and modifies nothing. Why is it okay for the first example but not this one: 我六点下班，以后，我去跟朋友吃饭?

2) Would removing the commas for the second example make it grammatically correct since 以后 would be then modifying 我六点下班? In other words, would this sentence -- without commas -- be correct: 我六点下班以后我去跟朋友吃饭. Would removing the comma here also make 以后 correct: 我们先复习一下，以后上新课?

"以后再也不来这里吃饭了，又贵又难吃" is grammatically correct because it does modify an event, and the event is 'from now on not come here for dinner'.

This sentence implies "從今(from now on)以后再也不来这里吃饭了，又贵又难吃"

'從今'(from now on) is omitted because we know '以后'(hereafter) is referring to 'from now, hereafter'

Just like if you wrote "(昨天在这里吃了一餐)以后，再也不會来这里吃饭了，又贵又难吃" 以后 modifies the event "昨天在这里吃了一餐".

2) Would removing the commas for the second example make it grammatically correct

我六点下班以后，我去跟朋友吃饭 is technically correct, but it should be a single sentence: "我六点下班(之后)去跟朋友吃饭"

Use 之后 instead of 以后 because 以后 is usually followed by a unique event

Example to illustrate my point:

清朝灭亡(以后), 中国再没有皇帝了 (correct)

清朝灭亡(之后), 中国再没有皇帝了 (correct)

'no more emperor' is a unique event, you cannot have this event again

雨停(以后)才能出外 (incorrect)

雨停(之后)才能出外 (correct)

'after the rain stop' is not a unique event, it will happen again and again

Best way is to simply write: "我六点下班(后)去跟朋友吃饭"

以后再也不来这里吃饭了，又贵又难吃

以后 in this context means from now on.

我六点下班以后，我去跟朋友吃饭

in this context, it means after. basically, ....以后 means after..... E.g. 十年以后: after ten years or ten years later.