below, be low

Below or below ( one word ) means beneath or under, literally or figuratively--
Put this on the shelf below the TV.
We were hoping to keep the cost below $1,000.
For the latest results, see the graph below.
His grades are below average.
Below also means beneath the deck of a boat--
The captain went below; he'll be back in aminute.

Be low or be low ( two words ) means to exist, take place or continue ( be ) close to the ground, or under a certain standard or limit ( low ), literally or figuratively--
The top shelf should be low, so it won't cover the window.
The volume should be low, so that the neighbors can't hear your phone messages.
His funds must be low; he won't have a drink with us.

Now that you know that, you can say--
"The lights must be low; if we can't dim them we'll put them below the stage."

Note--"bellow" means to yell or to make a loud sound like the "moo" of a cow--
Thye new sitter is all right, but I wish she wouldn't bellow at the children like that.