Mydearfellow,youhavebeeninvaluabletomeinthisasinmanyothercases,andIbegthatyouwillforgivemeifIhaveseemedTOPlayatrickuponyou.
Intruth,itwaspartlyforyourownsakethatIdidit,anditwasmyappreciationofthedangerwhichyouranwhichledmetocomedownandexamine
thematterformyself.HadIbeenwithSirHenryandyouitisconfidentthatmypointofviewwouldhavebeenthesameasyours,andmypresencewouldhavewarnedour
veryformidableopponentstobeontheirguard.Asitis,IhavebeenabletogetaboutasIcouldnotpossiblyhavedonehadIbeenlivingintheHall,andIremainanunknownfactorinthebusiness,readytothrowinallmyweightatacriticalmoment.
Butwhykeepmeinthedark?
Foryoutoknowcouldnothavehelpedusandmightpossiblyhaveledtomydiscovery.Youwouldhavewishedtotellmesomething,orinyourkindnessyouwouldhavebroughtmeoutsomecomfortorother,andsoanunnecessaryriskwouldberun.IbroughtCartwrightdownwithme--yourememberthelittlechapattheexpressoffice--andhehasseenaftermysimplewants:aloafofbreadandacleancollar.Whatdoesmanwantmore?Hehasgivenmeanextrapairofeyesuponaveryactivepairoffeet,andbothhavebeeninvaluable.

请教老师：

Foryoutoknowcouldnothavehelpedusandmightpossiblyhaveledtomydiscovery. 这一句，为何用了Foryoutoknow, 而没说Ifyouhadknown或者Hasyouknown...即从句为何没用虚拟语气呢？
用不定式复合结构Foryoutoknow的意思是什么？请和用Ifyouhadknown或者Hasyouknown.对比剖析。谢谢！

Foryoutoknowcouldnothavehelpedusandmightpossiblyhaveledtomydiscovery.

划线部分为主语。不定式复合结构作主语，表示这只不过一种可能性，而不是实质发生的事。主语的地方，你如何可以用一个条件状语从句呢？因为作主语的不定式短语隐含了与事实相反的假设，所以谓语动词用了虚拟语气形式。这种句子也是含蓄虚拟条件句。与事实相反的假设通过作主语的不定式来表达。假如要改为if 虚拟条件句，则主句需要用人称代词it指代if从句内容。比如：

If you had known, it couldnothavehelpedusandmightpossiblyhaveledtomy

discovery.