さ

The suffix さ is a versatile tool for nominalization. It functions similarly to '-ness' or '-ity' in English, representing the objective state or degree of a quality. Once nominalized, these words function as regular nouns and can take standard particles like が, を, and に.

1. Formation Rules:

• い-adjectives: Remove the final い and add さ.
  Example: 高い（たかい）　→ 高さ（たかさ） (height)
• な-adjectives: Add さ directly to the stem. Note that while さ can be added to many な-adjectives, it is less commonly used with highly noun-like ones because many are already nouns without な (e.g., 危険（きけん）), making the addition of さ redundant unless specifically emphasizing the degree.
  Example: 便利（べんり）　→ 便利さ（べんりさ） (convenience)
• Irregular Case: The adjective いい (good) uses its stem よ, becoming よさ (goodness). Compound adjectives where いい means 'good' also follow this rule (e.g., かっこいい becomes かっこよさ). However, regular adjectives that just happen to end in the い mora before the final い (like かわいい or おいしい) follow the standard rule (かわいさ, おいしさ). Note that when nominalizing an adjectival phrase containing a subject marker が, the particle changes to の (e.g., 頭がいい becomes 頭のよさ).
• Nouns: さ cannot be attached directly to nouns (e.g., 猫さ is incorrect). To nominalize a noun's qualities, you must first use a suffix like らしい or っぽい (see Section 3).

2. Nominalizing Negatives (ない → なさ):
When nominalizing the negative form ない of adjectives or certain states, it conjugates exactly like a regular い-adjective: simply drop the final い and add さ, resulting in なさ. Note that the negative forms of action verbs generally do not become standalone nouns with さ (using こと instead, like 食べないこと).
Example: 自信のなさ（じしんのなさ） (lack of confidence).
Example: 仕方ない（しかたない） → 仕方なさ（しかたなさ） (helplessness/lack of options).
Example: 頼りない（たよりない） → 頼りなさ（たよりなさ） (unreliability).

3. Desire and Auxiliary Adjectives:
さ can also attach to the たい form of verbs to express the intensity of a desire, or to patterns like やすい (easy to), にくい (hard to), らしい (typical of), and っぽい (-ish).
Example: 帰りたいさ（かえりたいさ） = the degree of wanting to go home. Note that 〜たさ is frequently followed by the particle に to express 'out of a desire to...' (e.g., 帰りたいさに = out of a desire to go home).
Example: 使いやすさ（つかいやすさ） = ease of use.
Example: 子供らしさ（こどもらしさ） = childlikeness.

4. さ vs. み:
While both nominalize adjectives, さ is objective and can be used with almost any adjective to describe a state or degree (e.g., 深さ（ふかさ）: depth in cm). In contrast, み is subjective and traditionally limited to specific words that describe a felt quality or state (e.g., 深み（ふかみ）: profundity/depth of character).
Note on slang: In modern Japanese internet slang and youth speech, み is frequently attached to other words (e.g., やばみ, わかりみ) to express subjective feelings, though this is not standard grammar.