想象句子是人体：主语 (头部) 告诉我们 “谁”，表语 (双脚) 描述 “什么状态/如何”。
核心规则:
系动词 (Be Verb) 就是 脊梁。头部不能直接连到双脚。
❌ 错误: "I happy" (骨折! 🦴)
✅ 正确: "I am happy" (连接成功! ✅)
Imagine a sentence is a human body. The Subject (Head) tells us Who, and the Complement (Feet) describes What/How.
Core Rule:
The Be Verb (Am/Is/Are) acts as the Spine. You cannot connect the Head directly to the Feet.
❌ Incorrect: "I happy" (Broken bones! 🦴)
✅ Correct: "I am happy" (Connected! ✅)