Why is 于 needed after 不屑 (but sometimes not)?

Question

In the following sentence:

也有一些“小资女人”的领军人物开始不屑于说英文了，偶尔来句日文或法文才能彰显自己的不同。

This sentence uses 不屑于, not 不屑, but the meaning would be something like:

Also, some leaders of “小资女人” started to look down on speaking English, and they sometimes think that to speak Japanese or French makes it clear that they are different from others.

^{The translation might be a bit awkward so feel free to edit it.}

I feel that the 于 here takes an object (说英文) after that. But in other usages, 不屑 just takes an object directly, without any preposition.

Or if you relate it to English, it looks like 不屑 here works as an intransitive verb, and 于 is a preposition. But in other uses I have seen, it works as if a transitive verb.

However, why does 不屑 need to take 于 here, even though in other cases it does not need?

note:https://baike.baidu.com/item/%E4%B8%8D%E5%B1%91/3178

Answer 1

不屑于 is a set phrase.

Check out《现代汉语规范词典》's distinctions:

不屑

1 动 不值得

不屑一切 | 不屑与之争辩

2 形 形容轻视的样子

一副不屑的神情。

and

不屑于

动 认为不值得做或不值得理睬

他很自负, 好像不屑于做这种具体工作。

《规范》defines the two slightly differently while treating 不屑于 as a separate independent word and not a construct.

Answer 2

不屑于说英文了

于, refer to the definition in the dictionary:

　2. 后缀（ａ．在形容词后，如“疏～防范”；ｂ．在动词后，如“属～未来）。

The sentence can be paraphrasing as:

对于说英文很不屑

In this case, better not omit 于, as 于 here specifies the thing you disdain 不屑.

Answer 3

“于” It is combined with nouns, pronouns, or nominal phrases to form a referential structure, serving as an adverbial or complement in a sentence. 1.The introduction of premises relating to the act. （1）Indicates where action occurs, occurs, or occurs. （2）The place or source of the introduction of an action.The object structure of “于” is used as a complement after a verb or predicate.It can be translated as "从（cong）", "自（zi）" to "由（you）" and so on. （3）The place where action is introduced.The object structure of “于” is used as a complement after a verb or predicate.It can be translated as "至（zhi）", "到（dao）" and so on. 2.Introduce time associated with action. （1）Indicates when an action occurs or occurs."于" can be used as an adverbial before the verb, or as a complement after the verb.Can be translated as in.Indicates when an action occurs or occurs."于" can be used as an adverbial before the verb, or as a complement after the verb.Can be translated into “在”. （2）Indicates when an action has been extended or terminated.The object structure of “于” is often used as a complement after the verb predicate.