To fall away is to become less in number.
As the night descended, the crowd began to fall away.
To fall back is to go back or to stop moving forward.
The enemy fell back.
Fall back on
To fall back on something is to use it when everything else has failed.
She knew that even if her acting career failed she had her MBA degree to fall back on.
To fall behind is to be slower than someone else.
The teacher asked him to bring his parents to school because he was falling behind in his schoolwork.
His landlady asked him to vacate the house because he was behind with the rent.
Fall down / fall down on
To fall down is to fail (in).
Don’t fall down on your job.
If something falls flat, it fails to produce the desired result.
Her joke fell flat.
To fall for something is to be deceived by it.
He fell for her charm. (= He was deceived by her charm.)
Fall for can also mean ‘fall in love with’.
He fell for his boss’s daughter.
Fall in with
To fall in with someone is to join them. To fall in with a plan or idea is to agree with it.
They fell in with our suggestion.
To fall off is to become smaller in number.
Attendance often falls off during the winter season.
To fall on somebody is to attack them.
The lion fell hungrily upon the deer.