If the B mt, put :
If the B . putting :
Don’t put : over a - der.
You’d be an * it.
(you have to have a southern speech impediment to say this right)
If the bee mount a’possum flea, pee your tea (ta ta)
If the bee space-out perioid, pee your titty in Gee space colon.
Don’t pee your tea (ta ta) overy dash d’period.
why, all you imposters feel Dee, space be space, and ass-trick it, period

okay…the real explanation?
If the B m t put :
If the B. putting :
Don’t put : over a - der
You’d be an * it
We were first informed of several “colonialisms.” In colonial times, the upper case letters were known as “Great” letters. Thus, the capital B is referred to as the “Great B.”
Next, we are informed that the period is known as a full-stop. Finally, all other punctuation is read by name.
With this explanation, we are able to read the ditty as:
If the great B m t put colon
If the great B full stop putting colon
Don’t put colon over a hyphen der
You’d be an asterisk it
or, with all spelling corrected:
If the grate be empty, put coal on,
If the grate be full, stop putting coal on,
Don’t put coal on over a high fender,
You’d be an ass to risk it.
Next spoiler please?