Learn Hardcore Japanese: akai ha medatu node wasurenikui desu. - A red book stands out, so it is hard to forget.

In Japanese, 赤い is an i-adjective and directly modifies the noun 本. Adjectives in Japanese don’t need a separate particle or connector when they sit before a noun. You simply attach the adjective in its attributive form (for i-adjectives that’s the same as the dictionary form) to the noun: 赤い本 “red book.”

Why is the topic marker は used after 本 instead of the subject marker が?

は marks the topic of the sentence (“as for red books”), setting up the context for what follows. Using が would identify 本 as a new or specific subject. Here we’re making a general statement about red books, so は is more natural. Contrast:

赤い本は目立つので… (As for red books, they stand out…)
赤い本が目立つと… (Red books stand out when…) ← Focuses on red books as the new information.

Why is the verb 目立つ in its plain (dictionary) form before ので rather than in its polite form or te-form?

When you connect a clause with ので (“because”), you attach ので to the plain (dictionary) form of verbs, i-adjectives, or nouns + な. Even in an otherwise polite sentence, the subordinate clause stays in plain form:
…目立つので忘れにくいです.
If you used polite stem 目立ち + ますので, it would sound odd and overly heavy.

What’s the difference between using ので and から to mean “because”? Why is ので chosen here?

Both ので and から can express reason, but they differ in nuance and structure:

ので is softer, more objective, and often used in explanations. It attaches to plain forms.
から is more direct and casual. It can attach to both plain and polite forms (e.g. 目立ちますから).
In this polite explanatory sentence, ので feels more natural and gently explains why red books are hard to forget.

What does the pattern 忘れにくい mean, and how is it formed from 忘れる?

忘れにくい is an i-adjective meaning “hard to forget.” It’s formed by taking the verb stem 忘れ (from 忘れる) + にくい, where にくい means “difficult to…” Together:
forget-stem + hard-to = “difficult to forget.”
This is a common way to turn verbs into adjectives that indicate difficulty:

食べる → 食べにくい (hard to eat)
見る → 見にくい (hard to see)

Why does the sentence end with です even though 忘れにくい is an i-adjective?

Adding です after an i-adjective makes the sentence polite without changing its core meaning. In casual speech you could simply say 忘れにくい. In polite speech you append です:
…忘れにくいです.
This is perfectly normal: i-adjective + です = polite style.