常用it作形式主语的句型结构
◇It+be+形容词+that-从句
可用于此句型的形容词有:wonderful,true，good，right，wrong，（un）important，useless，surprising，clear，unusual，lucky，certain,necessary等。如：
①Itisquitecertainthathewillbeatthemeeting.
②Itissurprisingthathecamelatetoschoolthismorning.
③Itisimportantthatwe(should)studyhard
注意:该句型中的形容词是（un）important,necessary等时，从句应为(should)+动词原形。◇It+be+-ed分词+that-从句
适用该句型的过去分词主要有:said,reported,announced,hoped,thought，told，believed，expected，decided，suggested,known等。如：
①Itishopedthatonedaytheywillhaveenoughanimalstosetthemfree
②Itiswellknownthattheearthgoesaroundthesun.
③Itissuggestedthatthesportsmeeting(should)beputoffuntilnextweek.
注意:该句型的过去分词是suggested时，从句应为(should)+动词原形。
◇It+be+名词+that-从句
适用该句型的名词（词组）有:apity，anhonor，agoodthing，afact，asurprise，goodnews,one’sduty等。如：
①Itisapity(that)youmissedthesportsmeetinglastweek.
②ItisafactthatEnglishisbeingacceptedasaninternationallanguage.
◇It＋seem／appear／happen的适当形式＋that从句。该句型常可以转换成含不定式的简单句。如：
①Itseemsthatthereisabigwastepipecomingdownfromthetown.
→Thereseemstobeabigwastepipecomingdownfromthetown.
②Heappearedtobecalm,butinsidehisheartwasbeatingwildlywithfear.
→Itappearedthathewascalm,butinsidehisheartwasbeatingwildlywithfear.
◇Itdoesn'tmatter(It’snowonder;Itdoesn’tmakemuchdifference等)
＋when/where/which/whether等从句。如：
①Doesitmatterifhecan'tfinishthejobontime?
②Theyareallclassmates.Itisnowonder
theyshouldhelpeachotherwiththeirstudies.
◇It+be+adj./n.(forsb./ofsb.)+todosth.该句型中的形容词通常是表示事物的特点或特征的如difficult，hard，easy，impossible，necessary，important等，此时用for；或表示人的性格特征或特点的如nice，good，bad，kind，silly，foolish，wise，clever，careless，rude，brave，cruel，careful，grateful等，这时要用of。如：
①Itisfoolishofyoutogiveupsuchagoodchance.
②Itisnecessaryforcollegestudentstomasteratleastaforeignlanguage
It作形式主语常见句型
动词不定式，动名词或主语从句作主语时，通常把它们放在谓语动词前，而用it作形式主语，放到句首，常见句型有：
1.It+be+形容词+主语从句，如：
ItisuncertainwhetherhecancometoJenny’sbirthdaypartyornot.
2.It+be+名词词组+主语从句，这类名词有：afact,apity,ashame,anhonour，aquestion等，如：
It’sapitythatyoumissedtheexcitingfootballmatch.Itremainsaquestionwhetherhewillcomeornot.
3.It+be+过去分词+主语从句，这类过去分词有：announced,believed,expected,hoped,decided,reported,said,shown等，如：Itisreportedthat16peoplewerekilledintheearthquake.Itisnotdecidedwhowillgivetheoperationtothepatient.
4.It+不及物动词(seem,appear,happen等)+主语从句,这类不及物动词有：appear,happen,seem等，如：
Itseemedthathedidn’ttellthetruth.
IthappenedthatIwasoutwhenhecalled.
5.It+be+形容词+forsb.）+动词不定式
这类句型常用形容词easy,difficult,hard,important,possible，wise等作表语，有时候为了强调不定式动作的执行者，常在不定式前加forsb，如：It’snecessaryfortheyoungtomastertwoforeignlanguage.Itisunwisetogivethechildrenwhatevertheywant.
这类形容词有：easy,difficult,hard,important,possible，wise
6.It+be+形容词+ofsb.+动词不定式，这类形容词常是表示心理品质,性格特征的形容词，如：kind,nicestupid,clever,foolish,polite,impolite,silly,selfish,considerate等。某人和这些形容词可以构成主系表结构。如：
It’sverykindofyoutohelpmewiththework.
=Youarekindtohelpme.
Itseemedselfishofhimnottosharehisdictionarywithothers.=Heseemedselfishnottosharehisdictionarywithothers.
7.It+be+名词词组+动词不定式，如：
Itisnotagoodhabittostayuptoolate.
8.It+be+名词或形容词+动名词，这类名词和形容词常常是：good,nogood,nouse,awasteof,useless,senseless等，如：
It’sawasteoftimetalkingtoheranymore.
Itisnousearguingaboutthematterwithhim.
9.It+take(sb.)+时间（金钱）+动词不定式，如：
Ittooktheworkersalmostthreeyearstofinishbuildingthedam.Itwilltakeawholedaytogettothetopofthemountainonfoot.
