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. （it形式主语，主语从句是真正主语）
谓语（predicate）: 说明主语的动作、状态和特征。
we study english. he is asleep.
表语（predicative）: 系动词之后的成分，表示主语的性质、状态和特征。
he is a teacher. （名词） seventy-four! you don’t look it. （代词）
five and five is ten. （数词） he is asleep. （形容词）
his father is in. （副词） the picture is on the wall. ( 介词短语)
my watch is gone / missing / lost. （形容词化的分词）
to wear a flower is to say “i’m poor, i can’t buy a ring. （不定式）
the question is whether they will come. （表语从句）
（常见的系动词有: be, sound（听起来）, look（看起来）, feel(摸起来，smell（闻起来）, taste（尝、吃起来）,
remain（保持，仍是）, feel（感觉） ...
it sounds a good idea. the sound sounds strange.
her voice sounds sweet. tom looks thin.
the food smells delicious. the food tastes good.
the door remains open. now i feel tired.
宾语：1）动作的承受者-----动宾
i like china. （名词） he hates you. （代词）
how many do you need? we need two. （数词）
we should help the old and the poor. i enjoy working with you. （动名词）