定義：
不定式（infinitive）有兩種形式：1. 帶to的不定式（to-infinitive）；2. 不帶to的不定式（bare infinitive）。後者即通常所謂的動詞原形。
最常用的不定式用法：
1. 表動作的目的或意圖（to的意思同「in order to」或「so as to」）；
*She's gone to collect her pay cheque.
她去收集她的支付支票。
*The three bears went into the forest to find firewood.
這三隻熊到森林裡去找木材。
2. 作為句子的主題：
*To be or not to be, that is the question.
生存還是毀滅，這是個問題。
*To know her is to love her.
了解她才是愛她。
3. 連接名詞或代詞，表示什麼東西用於什麼用途，或這是要做什麼：
*Would you like something to drink?
你想喝點兒什麼？
*I haven't anything to wear.
我沒什麼衣服穿。
*The children need a garden to play in.
這孩子需要一個花園在裡面玩兒。
4. 在這些句型中，跟在形容詞之後：
①It is + adjective +to-infinitive
*It is good to talk.
這是很好的交談。
②It is + adjective + infinitive + for someone + to-infinitive.
*It is hard for elephants to see mice.
對於大象來說，很難看到老鼠。
③It is + adjective + infintive + of someone + to-infinitive.
*It is unkind of her to say that.
她很刻薄地說了那些話。
5. 當評論或判斷時，放在形容詞+名詞後：
*It was a stupid place to park the car.
這是一個愚蠢的停車地方。
*This is the right thing to do.
這是正確的做法。
*It was an astonishing way to behave.
這是一個驚人的表現方式。
6. 在下面這些句型中，與too和enough連用：
①too much/many (+noun) + to-infinitive
*There's too much sugar to put in this bowl.
碗裡放了太多糖。
②too + adjective + to-infinitive
*This soup is too hot to eat.
湯太燙沒法喝。
③too + adverb + to-infinitive
*He arrived too late to see the actors.
他來得太晚了，沒看到演員。
④enough (+ noun) + to-infinitive
*I've had enough (food) to eat.
我已經吃得夠多了。
⑤adjective + enough + to-infinitive
*She's old enough to make up her own mind.
她已經夠大了，能自己做決定了。
⑥not enough (+noun) + to-infinitive
*There isn't enough snow to ski on.
沒有足夠的雪來滑雪。
⑦not + adjective + enough + to-infinitive
*You're not old enough to have grand-children!
你還沒有老到有孫子呢。
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