grammar → 是
TSUMUGU · TBCL 1 (est.) · 語法
grammar point · tier 1 · 繫動詞 — copula 是 (link a noun to a noun)
· shì
Copula: sets two nouns equal — X is Y. Links a noun to a noun for identity or category, never a noun to an adjective.

字源 FORM what the parts do

The sun (日) above, straight (正) below — what stands true under the sun. The classical pointer 'this' settled into the graph and became the everyday verb to be.

故事 STORY a scene to remember it by

Two things laid side by side under one true sign, and the sign holds them as the same one.
字源記憶法
框 · Frame
[noun A] 是 [noun B]
觸 · Trigger
You want to say one thing is another thing — name it, or place it in a class.
序 · The move
1name the first thing (noun A)is A a noun or pronoun?
2是 sets the equals signare both sides nouns?
3name the second thing (noun B)is B a noun, not an adjective?
4for an adjective predicate, drop 是 and reach for 很 insteaddid an adjective sneak in after 是?
例 · Examples
1Iam學生student
I am a student.
界 · Boundary
很 (adjective predicate)
是 joins noun to noun (我是學生). An adjective takes 很, not 是: 我很高, never 我是高. The thing after 是 must be a noun.
是…的
Bare 是 equates two nouns. 是…的 closes 的 behind a detail to spotlight one fact of a past event (我是昨天到的).
我是高 → 我很高
這本書是貴 → 這本書很貴
他是忙 → 他很忙
English 'to be' covers both identity and adjectives (I am a student / I am tall), so learners over-insert 是 before adjectives where Chinese takes 很.