你的五句话：
1.Not until his mom came back did he go to bed.
强调句是 It's not until his mom came back that he went to bed.
还有：He didn't go to bed until his mom came back
2.It's Tom who saved the old man at expense of his own life.
3.It is because of the reason known to all that American diet is majorly made of meat and potatoes.