系動詞連接了主語和補語。這種結(jié)構(gòu)中的補語是說明主語性狀的，所以稱為主語補足語。系動詞作為實義動詞的一類，具有本身的意義，但是在起到橋梁作用的同時，有些系動詞會保持本身具有的意義，有些意義則會減弱或消失。例如：
　　1. I tasted the soup. ( taste 為及物動詞，意思為“品嘗”。)
　　2. The soup tastes wonderful. (taste 為系動詞，意思為“嘗起來，吃起來”，意思基本未變。)
　　3. He went to the cinema yesterday evening. ( go 是不及物動詞，意思為“去，走”。)
　　4. At the news, he went mad. (go為系動詞，失去了“go”的原義，意為“變得”。)
　　另外，英語中還有一定數(shù)量的不及物動詞，完全保留了本身的詞匯意義，同時又發(fā)揮著系動詞的功能。它們與主語補足成分連用，所以這類詞被稱為“準(zhǔn)系動詞”(quasi-linking verb or half-linking verb)。用于“S+Vi+Cs”(S代表主語，Vi代表不及物動詞，Cs代表主語補足語)這一句式的動詞都屬于這種用法。有時，主語補足語不緊跟系動詞，有可能被其它狀語成分隔開。以“go”為例：
　　5. At twenty-four, Tom went half-bald. ( go 為系動詞，失去原義。)
　　6. After midnight, Tom went home bored.( go 為準(zhǔn)系動詞，保留原義，但同時又連接主語“Tom”和主補“bored”，“home”為副詞，表示地點狀語。)
　　用于這一句式的補語可以為各種形式，如名詞詞組(NP)，形容詞詞組(AP)，介詞詞組(Pr.P)，和分詞短語(PP)等。現(xiàn)舉例如下：
　　Type1: S + Vi +Cs (NP)
　　7. We parted the best friends.
　　= We were the best friends when we parted.
　　8. I stand before you today the representative of a family in grief.
　　= I am the representative of a family in grief when I stand before you today.
　　類似的動詞包括： come (back), die, fall, go, leave 等。
　　Type2: S + Vi +Cs (AP)
　　9. They were born poor, lived poor, and poor they died.
　　= They were poor when they were born; they were poor when they lived; they were poor when they died.
　　10. She married young.
　　= She was young when she married.
　　11. The morning dawned fresh and clear after the storm at night.
　　= When the morning dawned after the storm at night, it was fresh and clear.
　　類似的動詞包括：arrive, come (back), dawn, fall, leave, lie, marry, return, sit, stand, turn out 等。
　　Type3: S + Vi +Cs (Pr.P)
　　12. The parcel arrived in good condition.
　　= When the parcel arrived, it was in good condition.
　　13. They separated with feelings alienation.
　　= They had feelings alienation when they separated.
　　類似的動詞包括：come, die, fall, go, leave, lie, part, return, sit, stand等。
　　Type4: S + Vi +Cs (P.P)
　　14. They stood listening to him.
　　= They stood while they were listening to him.
　　15. He came home convinced that she was telling the truth.
　　= He came home and he was convinced that she was telling the truth.
　　類似的動詞包括：arrive, die, fall, go, leave, lie, return, sit 等。