What's the difference 随分{ずいぶん}, 大分{だいぶ} vs 非常{ひじょう}に?


Are there any differences between these three words, or are they synonyms?
I've tried asking on HiNative and searching for it on that site, as well as
here, but couldn't find any answers.




These three adverbs have pretty similar meanings so one may say that they are
synonyms. But they are not always interchangeable as their usage differs a
bit:

随分{ずいぶん} means “very,” “extremely,” “awfully” or “certainly,” and is used when
the speaker wants to express that the degree of something is higher than they
were expecting.

> E.g:この荷物{にもつ}はずいぶん重{おも}いですね。
>
> This luggage is extremely heavy, isn’t it? (Connotation: Did not expect the
> luggage to be so heavy.)

大分{だいぶ} means “considerably/quite/very” and shows that the degree or quantity
of something is higher than average.

> E.g.今日{きょう}はだいぶ暖{あたた}かい。
>
> It is pretty warm today. (Connotation: The warmth is higher than avarage)

非常{ひじょう} means "outstanding/unusual/not average" and shows that something is
not ordinary, often approaching an adverb like "very". Note also that this
adverb is very strong, and is common to see it in the negative sense.

> E.g.非常{ひじょう}に困{こま}っています。
>
> I am in deep waters. (Expression: I am very troubled.)




非常に is much stronger than the other two. It's a formal word that means "very"
or "highly".

随分 and 大分 are often interchangeable, and they both roughly mean "fairly",
"pretty", "a lot", etc. The difference is that 随分 tends to have a connotation
of "unexpectedly" or "more than expected". Because of this, you usually do not
use 随分 for something that is completely under your control.

For example, 体調は大分良くなりました and 体調は随分良くなりました are interchangeable (you cannot
control your physical condition perfectly). 宿題は大分出来ました is perfectly fine, too
(roughly 70-80% is completed). However, 宿題は随分出来ました is a nuanced sentence like
"I didn't expect this but (somehow) I made a progress a lot".

