akai hon ha medatu node wasurenikui desu.

In Japanese, adjectives directly modify nouns with no particle in between.

• 赤い + 本 = 赤い本 (a red book)
  赤い is an い-adjective, and the plain (dictionary) form can modify a noun like this.

ので means because / since, connecting a reason to a result:

• 目立つので忘れにくい = Because it stands out, it’s hard to forget.

Compared to から:

• ので sounds a bit softer / more explanatory / more polite (often used in polite speech, formal writing, giving reasons without sounding blunt).
• から can sound more direct or more like a personal justification.

With ので, you typically attach it to the plain form of the clause:

• Verb plain: 目立つ + ので
• い-adjective plain: 高い + ので
• Noun/な-adjective: 静か + なので, 学生 + なので

You generally don’t insert です before ので in a normal clause. (There is ですので, but it’s a separate, more formal connective meaning therefore / for that reason, and it’s used differently.)

〜にくい attaches to the masu-stem of a verb to mean hard/difficult to do:

• 忘れる (to forget) → stem 忘れ → 忘れにくい = hard to forget

It describes difficulty due to circumstances (not ability/skill). Here: it’s hard to forget because it stands out.

They’re not the same:

• 忘れにくい = hard to forget (difficult for it to slip your mind; likely you won’t forget)
• 忘れられない = can’t forget (often emotionally strong: I can’t forget it, unforgettable)

This sentence is talking about memorability/noticeability, so 忘れにくい fits well.

です here is the polite sentence ending.
You can say:

• Casual: 赤い本は目立つので忘れにくい。
• Polite: 赤い本は目立つので忘れにくいです。

With い-adjectives, you can add です to make the tone polite without changing the core meaning.