1. stab sb in the back
stab sb in the back指的是在（某人）背后捅刀子（指伤害信任自己的人）。
He had been lied to and stabbed in the back by people that he thought were his friends.
他曾经被一些他视为朋友的人欺骗过，在背后被捅过刀子。
 
2. draw the line
draw the line可不是“画一条线”，它的意思是是“绝不做”。
I swear a lot, but even I draw the line at saying certain words.
我经常说脏话，但即便如此有些话我绝不会说。
 
3. flatter yourself
flatter yourself有“自命不凡，自以为是”的意思，后面可以搭配that从句：
Steve flatters himself that he's an excellent speaker.
史蒂夫自认为是个优秀的演说家。
flatter to deceive的意思是“以假象蒙人，掩人耳目”。
I suspect these statistics flatter to deceive.
我怀疑这些数据是掩人耳目的。
be/feel flattered就是被夸奖了，形容“被奉承得高兴；感到荣幸”。
She was flattered by his attention.
他的关注使她格外高兴。
 
4. in a good mood
mood表示心境，心情，情绪；精神状态。
be in no mood for sth/to do sth意思是全然不想…／不想做…。
I was in no mood for chatting.
我没有心情聊天。
而be in the mood (for sth/to do sth)的意思是“有意（做／要…)”。
We could go out to eat, if you're in the mood.
如果你想的话，我们可以出去吃饭。