A snake in the grass is a pretentious, dangerous person such as a hypocrite, mole, or spy. This idiom draws on the danger of being bitten by an unseen snake in tall grass, likely because it was stepped on or felt threatened.
"Take heed" is a warning to pay attention, similar to "beware" or "be careful." Compare "Therefore let him who thinks he stands take heed lest he fall." (1 Cor 10:12, NKJV).
Definitions for both idioms, from The Free Dictionary, are linked to below.
Answer
There are plenty of people around like this. You have to be careful who you trust, believe me, I know.
A False friend is just like a snake in the grass.
A "snake in the grass" is a person that should be regarded with suspicion because they are not trustworthy. An example of this idiom used in a sentence would be, "Natalie knew not to trust Nathan, he was a snake in the grass which would strike at the first available opportunity."
traitor, backbiter
The rattle snake lives in the grass.
Think about that for a minute. What would happen if you held a snake close to your chest? You'd probably get bitten sooner or later. If you nurture a snake, you've cared for it, and then it turns around and bites you. This idiom means you've trusted someone close to you and they betrayed you.
It's only a grass snake
It means that you are the lowest of the low. You can't be any lower.
yes because the grass snake is bigger.
In the sentence the slimy, green snake slithered through the tall grass the nouns are snake and grass
Usually this idiom is used in a battle or military application in which you remove the most dangerous opponent or the leader first, so that the other enemies will be easier to deal with.
I thought he was a friend, but he turned out to be a real snake in the grass.
The simple answer is you don't ! The Grass-snake, along with the European Adder and the Smooth Snake (which is actually a lizard) are ALL protected species.