are

v. 是

Don't cry, you are a big boy now.
别哭，你现在已经是大男孩了。
These people who were young fifty years back are now gray haired.
50年前的年轻人，现在已经白发苍苍了。
Dark clouds are a sign of rain or snow.
乌云是雨或雪将至的预兆。
There are 300 students or so in this middle school.
这所中学大约有300个学生。
Love and hate are extremes.
爱和恨是两个极端。
There are more than 5,000 adjectives in that dictionary.
那本词典中有5000多个形容词。
You are just saying that.
你只是说说而已。
Who do you think you are?
你以为你是谁？

无

名词 are:

a unit of surface area equal to 100 square meters
同义词：ar
动词 be:

have the quality of being; (copula, used with an adjective or a predicate noun)
同义词：be
be identical to; be someone or something
同义词：be
occupy a certain position or area; be somewhere
同义词：be
have an existence, be extant
同义词：exist, be
happen, occur, take place
同义词：be
be identical or equivalent to
同义词：equal, be
form or compose
同义词：constitute, represent, make up, comprise, be
work in a specific place, with a specific subject, or in a specific function
同义词：be, follow
represent, as of a character on stage
同义词：embody, be, personify
spend or use time
同义词：be
have life, be alive
同义词：be, live
to remain unmolested, undisturbed, or uninterrupted -- used only in infinitive form
同义词：be
be priced at
同义词：cost, be