三级。分别是原级，比较级，最高级。例如：
good→better→best
happy→happier/ more happy→happiest/ most happy
excited→more excited→most excited
tall→taller→tallest
bad→worse→worst
low→lower→lowest
fascinating→more fascinating→most fascinating
friendly→more friendly→ most friendly
You are taller than me.
She is the youngest of you all.
