不定式定義：
由to+動(dòng)詞原形構(gòu)成。不定式是一種非限定性動(dòng)詞。而非限定動(dòng)詞是指那些在句中不能單獨(dú)充當(dāng)謂語的動(dòng)詞，可分為不定式，動(dòng)名詞，現(xiàn)在分詞和過去分詞。
“動(dòng)詞不定式”由動(dòng)詞+不定式構(gòu)成。動(dòng)詞不定式在句中可以作句子除謂語之外的任何句子成分。動(dòng)詞不定式的被動(dòng)形式除了一般形式外還有其完成式和進(jìn)行式。
常見的有：like / love，want，need，ask，help等。
例：I like to play with Tom. 我喜歡和湯姆玩。
I want to play with Tom. 我想和湯姆玩。
I like to eat dumplings. 我喜歡吃餃子。
動(dòng)詞不定式一般結(jié)構(gòu):
疑問詞who,what,which,when,where,whether,how后可接不定式構(gòu)成不定式短語，在句中作主語、賓語、表 語等。如：
�、賅hen to leave for London has not been decided yet. （不定式在句子中做主語）
�、贛r. Smith didn't know whether to leave or stay there. （不定式在句子中做賓語）
�、跧 asked Professor Xu how to learn English well. （不定式在句子中做直接賓語）
�、躎he question was where to get the medicine needed. （不定式在句子中表語）
以上例句中疑問詞+不定式部分，均可轉(zhuǎn)換為相應(yīng)的從句形式。如：
�、賅hen we shall leave…
�、邸環(huán)ow I could learn…
經(jīng)常在這種結(jié)構(gòu)中使用的動(dòng)詞有：
consider,decide,discover,explain,find out,forget,hear,know,lea rn,observe,understand,wonder等。
動(dòng)名詞與不定式的區(qū)別:
1 動(dòng)名詞與不定式的區(qū)別：
動(dòng)名詞表達(dá)的是：狀態(tài)，性質(zhì)，心境，抽象，經(jīng)常性，已發(fā)生的
不定式表達(dá)的是：目的，結(jié)果，原因，具體，一次性，將發(fā)生的
2 接不定式或動(dòng)名詞，意義相同。
3在下列情況下，一般要用不定式：
�、賖ate,like,love前有would(should)時(shí)，
如：I'd like to have a cup of coffee.
�、诋�(dāng)謂語動(dòng)詞begin,continue,start等是進(jìn)行式時(shí)，
如：Thestudents are starting to work on the di fficult maths problem.
�、踒egin,continue,start與know,understand等狀態(tài)動(dòng)詞連用時(shí)，
如：I soon began to understand what was happening.
�、�.advise,allow,encourage,forbid,permit等動(dòng)詞后接動(dòng)名詞作賓語，或帶不定式作賓語補(bǔ)足語。如：
Our teachers don't permit our/us swimming in the lake.
Our teachers don't permit us to swim in the lake.
4 部分動(dòng)詞后接不定式或動(dòng)名詞時(shí)，意義差別較大，應(yīng)根據(jù)句子語境選擇使用。
forget,remember,regret后接不定式，表示現(xiàn)在或未來的動(dòng)作，接動(dòng)名詞表示動(dòng)作已經(jīng)發(fā)生。如：
Don't forget to post the letter for me.
Have you forgotten meeting her in Beijing Airport?
Remember to close the windows before you leave.
I remember writing him a letter a year ago.
We regret to tell you that all of you are not invited toattend the meeting.
They regretted ordering these books from abroad.
省to的動(dòng)詞不定式:
�。�1）情態(tài)動(dòng)詞( 除ought 外，ought to意思是“應(yīng)該”，是情態(tài)動(dòng)詞，只有一種形式，后邊接動(dòng)詞不定式，to不能省略。
ought to沒有人稱和數(shù)的變化，后接動(dòng)詞原形可以表示現(xiàn)在、將來或過去將來，由時(shí)間狀語或上下文決定。例如：
They ought to come tomorrow.他們明天應(yīng)當(dāng)來)：
�。�2）使役動(dòng)詞 let,have,make：
（3）感官動(dòng)詞see,watch,look at,notice,observe,hear,listen to,smell,feel,find 等后作賓補(bǔ)，省略to。
注意：在被動(dòng)語態(tài)中則to 不能省掉。
在使役動(dòng)詞中g(shù)et除外(get sb. to do sth.)
I saw him dance.
=He was seen to dance.
The boss made them work the whole night.
=They were made to work the whole night.
（4）表示個(gè)人意愿或傾向的would rather，had better，might(just) as well：rather than置于句首時(shí)。
Rather than ride on a crowded bus,he always prefers to ride a bike.
�。�5）Why… / why not…：
（6）help 可帶to，也可不帶to,help sb (to) do sth：
�。�7）but和except：but前是動(dòng)詞do時(shí)，后面出現(xiàn)的動(dòng)詞用不帶to的動(dòng)詞不定式。
�。�8）由and,or和than連接的兩個(gè)不定式，第二個(gè)to 可以省去：
�。�9）通常在discover,imagine,suppose,think,understand等詞后，可以省去to be：
He is supposed (to be) nice. 他應(yīng)該是個(gè)好人。
（10）but作介詞，后接不定式結(jié)構(gòu)時(shí)，前面謂語動(dòng)詞部分若含有do的形式時(shí)，but后的不定式要省去to，否則要帶to。
He wants to do nothing but go out.
He wants to believe anything but to take the medicine.
�。�11）當(dāng)兩個(gè)或多個(gè)不定時(shí)短語由連詞and，but或or連接時(shí)，后一個(gè)或幾個(gè)不定式符號(hào)to常省略。但若表示對(duì)比、對(duì)照關(guān)系時(shí)，則不能省略。
He wants to move to France and marry the girl.
The purpose of new technologies is to make life easier,not to make it more difficult.
（12）不定式做表語時(shí)，一般要帶to，但若主語部分中含有do的各種形式時(shí)，符號(hào)to可省去。
We've missed the last bus.All we could do now is walk home.