1.No, technically I have six, and I'm mulling over a gum purchase, so I think you should go.

【句子翻译】严格地说，我买了六瓶。我还在琢磨要不要买口香糖，所以你先吧。

【句子解析】mull over 认真琢磨；

He tried to mull things over in his mind.
他试图在他的脑海中思索事情。

2.This is the most exciting thing that ever happened to me.

【句子翻译】这竟是我有生以来最高兴的事。

【句子解析】这是个包含定语从句的复合句，that引导的从句修饰thing，由于前面有最高级most修饰thing，所以后面要用that而不是which来引导；