When something is important or required, we say that it’s necessary. You need to do something, and if you don’t do it, there could be a negative consequence:
It’s necessary to learn English if you live in the United States or do business with a company in the United States.
Filling out this paperwork is necessary if you want to get your rebate. (rebate = refund)
Is this necessary? Do I need to do this?
I don’t think this is necessary.
It’s necessary to wear a heavy coat during the winter.
She’s wearing a heavy coat.
The word "necessary" is an adjective. It’s often used with the negative form of the verb "be."
We found out that it wasn’t necessary to do all that work.
These extra options for the car aren’t necessary, so let’s not get them.
A coat won’t be necessary today. It’s going to be warm.
All that salt in the recipe isn’t necessary. You can reduce it by half.
The adverb form is a little difficult to use and pronounce. In a sentence it looks like this:
She’s wearing a heavy coat.