Idioms
The grass is always greener on the other side.
This sentence is an idiom. An idiom is a figure of speech established by usage that has a meaning which is not necessarily clear from the words used. Idioms are usually used to express a particular sentiment.
The grass is always greener on the other side.
This idiom does not literally mean that the other side will always have greener grass. There may not even be a literal other side to the subject at hand, or grass, for that matter.
The meaning of this idiom is that people think the other person has something better than they do.
Idioms are very close in nature to metaphors, and the most efficient way of studying idioms is to classify them by subjects:
Idioms about NATURE
down to earth - without pretensions
over the moon - very happy
out of the blue - complete surprise
the tip of the iceberg - a very small part of something very big
Idioms about SPORT
out of your league - above your head
the ball is in your court - you need to respond
he is the front runner - in the lead
that's par for the course - something to be expected
Idioms about FOOD
that was a piece of cake - something very easy
crying over spilt milk - regretting something too late
not my cup of tea - something you are not interested in
as easy as apple pie - something very easy
Idioms using the word KEEP
keep your cool - be relaxed
keep an eye on - pay attention to
keep track of time - not to be absent-minded
keep your fingers crossed - hope for good luck
They are widely tested in CAE keyword sentence transformation exercises.