He _______(not fall) asleep until his parents came back.

not to fall

had not fallen

答案:空白处应填:not to fall
解析：try有两中搭配：try to do sth.和ty doing sth.
try to do sth的意思是”试图,努力做某事”题中正是此意：tom努力不让自己落后于他人．不定式的否定形式是”not to do sth."
try doing sth.的意思是”尝试做某事”