1,Johnsaidhismotherwouldbuyhimafive-speedsracingbicycleforhissixteenthbirthday.

　　像five-speeds这种数词+名词构成的合成词或词组作另一个名词的定语是,只能用单数形式,所以要改为：five-speedracingbicycle

　　2,Byreadingthedirectionscarefully,somemistakesintheexaminationcouldhavebeenavoided.

　　此题中的couldhavebeenavoided是虚拟语气,而本句所表达的意思并非与事实相反,by...表示“通过...方法”,那么其后的“避免”是一个必然的事实,因此应改为：couldbeavoided,

　　3,Ican'thelptothinkitwouldbefuntoplaysuchanexcitinggamewiththem.

　　can'thelp（禁不住）后面的非谓语动词宾语要求动名词,应改为can'thelpthinking

　　4,Ofthetewlectures,thefirstwasbyfarthebestone,partlybecausethepersonwhodeliveredithadsuchapleasantvoice.

　　既然是twolectures,就只能用比较级,所以byfarthebestone中的thebest应该为better.

　　5,Thechairmanproposedthateveryspeakermustlimithimselftofifteenminutes.

　　本句表示主语（Thechairman）的愿望,因此后面的宾语从句要用虚拟语气,因此mustlimit应该把must去掉.

　　6,WhenIgottothecinema,thefilmhadalreadystarted;Ioughttogetthereearlier.

　　Ioughttogetthereearlier是前面句子并列的主句,时态应保持一致,因此应该是Ioughttohavegotthereearlier.

　　7,Aftercriticizingourproposalineverydetail,thedirectorwentontellingusthattherewasnothingpersonalinhiscomments.

　　criticizingourproposal与tellingus时两件相接的不同话题,而goondoing表示的是继续同一个话题,因此应该是goontotell,即接下来继续另一个话题.