distinguish between grammar patterns and how to remember them


Suppose we take おそれがある, we can use it with either noun or a verb:

> V る ＋ おそれがある  
>  N ＋ の ＋ おそれがある

and on the other hand if we take に違いない, we can use it with
everything(noun,adjective and adverb)

Plain Form Whatever ＋ に違いない must be [whatever]; (I'll)

I feel that somehow I can also construct おそれがある along with an adjective but
the sentence will be grammatically wrong as it's mentioned that おそれがある can
only be used with nouns and verbs.My question is, how can I remember which
grammar forms I can use with noun-verb or noun-adjective-verb or only verb ??
Do I have to memorize everything or is their any logic




Good question! My gut was telling me an isolated adjective would not work with
恐れがある. Checking a formal definition, it indicates "fear that something
unfavourable might happen":

[悪いことが発生するのではないかと推測されるさまを表す語。](https://www.weblio.jp/content/%E6%81%90%E3%82%8C%E3%81%8C%E3%81%82%E3%82%8B)

And I think this is the crux of it:

**恐れがある is preceded by an event.**

A noun or verb may readily designate an event, but an adjective would have to
describe one in order to fit.

> 大雨{おおあめ}の恐れがあります。 There is a risk of heavy rain.
>
> この暗号{あんごう}方法{ほうほう}は破{やぶ}られるおそれがある。 I fear this encryption method may get
> cracked.
>
> 交渉{こうしょう}は激{はげ}しい恐れがございます。 We fear the negociations might turn out to be
> heated.

