be + behind
We are behind.
We're six months behind with the rent.

fall + behind (1)
I fell behind in my car payments.

fall + behind (2)
The company fell behind competitors who had more attractive merchandise and better prices.

get + behind (1)
We want to help you avoid getting behind with your rent.

get + behind (2)
Let's all get behind John for president.

leave + behind (1)
I left my briefcase behind in the train.

leave + behind (2)
In statistics, Cathy leaves everyone else behind.