句子成分主语（subject）: 句子说明的人或事物。The sun rises in the east.
（名词）
He likes dancing.
（代词）Twenty years is a short time in history. (数词)Seeing is believing.
（动名词）
To see is to believe.
（不定式） What he needs is a book. （主语从句）It is very clear that the elephant is round and tall like a tree.（...
