•Thorny (adj.) -

A thorny problem or subject is difficult to deal with. （棘手的，麻烦的）

Examples:　　

1.It is essential that we tackle this thorny problem. 

2.The thorny issue of illegal immigration remains unresolved.

•Take the lead (in doing sth) (idiom) -

To be the first to start doing something as an example for others to follow.（率先，带头）

Examples:　　

1.His company has taken the lead in developing this new technology. 

2.The U.S. took the lead in declaring war on terrorism.

•Feel blue (idiom) -

Be depressed or sad.（压抑，伤感）

Examples:　　

1.You look like you feel blue. What's wrong?  

2.I have been feeling blue lately, probable because of this gloomy weather. 

•Be suited for (sth) (idiom) -

To be fitting or appropriate for something. （适合…，与…相配）

Examples:　　

1.Do you think I am suited for this kind of work? 

2.Those clothes are not suited for outdoor work.

•The crux (of sth) (noun) -

The most important aspect of something.（关键，症结）

Examples:　　

1.The crux of the matter is that people are afraid of change.

2. The crux of their economic problems is inflation.