句子中的谓语动词(predicative verb)是限定的;限定动词受人称(person)和数目(number)的限制。此外，还有时态(tense )、语态(voice)和情态(mood)等。例如：

　　① Good leaders serve the people whole-heartedly.

　　② Jason is giving a public lecture.

　　③ Who did it?

　　④ I would appreciate it if you could do me a favour.

　　在句子中可以施展其他功能的动词是非限定动词;这些动词具有一般动词的许多特性，但没有“人称”和“数目”的限制。

　　非限定动词分三种：不定式动词(the infinitive); 分词( the participle); 动名词(the gerund); 分词又分现在分词( the present participle)和过去分词(the past participle)。

　　例如：

　　⑤ Tom found the prescribed text hard to read.

　　⑥ Helen has a smiling face.

　　⑦ Your written English is good.

　　⑧ Smoking is harmful to health.

　　⑤里的是不定式动词;⑥里的是现在分词;⑦里的是过去分词;

　　⑧里的是动名词。

　　非限定动词及其短语，语法功能庞大，涉及下列五方面：

　　㈠ 当主语

　　不定式动词和动名词都可当句子的主语，取代名词和代名词。例如：

　　⑨ To swim in a warm day is joyful.

　　⑩ Swimming makes us hale and hearty.

　　㈡ 当补足语

　　可当主语的句子成分也可当补足语。例如：

　　● A good government"s desire is serving the people.

　　● What is difficult is to hoodwink the public indefinitely.

　　同样的，现在分词与过去分词也可充当接系动词(the linking verb)的补足语。例如：

　　● The witness"s evidence is quite convincing.

　　● At long last, John was fully convinced.

　　㈢ 当宾语

　　不定式动词和动名词都可充当及物动词的宾语。例如：

　　● The baby wanted to drink milk.

　　● Janet suddenly stopped talking.

　　它们也可以成为介词的宾语，但除了“except, but, about, than”之外，介词的宾语多数是动名词。

　　例如：

　　 I am tired of repeating the same point.

　　 Most of us desired nothing except to succeed.

　　 Betty did nothing useful but talk(不带”to”)。

　　 That weak old man is about to die.

　　㈣ 当名词修饰语

　　 Don"t disturb the sleeping dog.(现在分词)

　　 Can you remove the damaged machine?(过去分词)

　　 That is the only thing to do.(不定式动词)

　　 The sailors came in a fishing boat.(动名词)

　　㈤ 当动词修饰语

　　 Walking along the road, he came across a friend.(现在分词短语)

　　 Extremely frightened, the picnickers fled.(过去分词短语)

　　 The athlete stopped to take a rest?(不定式动词短语)

　　 Peter came running.(动名词)