1.(Doing) sth is a must.
Warm clothes are a must in the mountains.
2.preserve
sth that is only suitable or allowed for a particular group of people.
America used to be the land of opportunity and optimism. Now opportunity is seen as the preserve of the elite.
3.be worth (doing) sth
值得做某事
The small town is worth visiting.（be worth doing）
The small town is worth a visit.（be worth sth）
I think it to be a shame.
I think that it is a shame.
I think it is a shame.
I think it a shame.
They may think it to be worth sacrificing A for B.
They may think that it is worth sacrificing A for B.
They may think it is worth sacrificing A for B.
They may think it worth sacrificing A for B.
4.drawback
disadvantage, downside, pitfall, flaw,weakness, problem.
