I don't know any rules of thumb here, but the two verbs don't have much to do with each other except in two special uses. In the old nursery rhyme, \"Little Bo Peep,\" Ms. Peep was urged to \"leave them [the sheep] alone, and they'll come home.\" Burchfield notes that that rhyme was largely responsible for \"leave\" being used where \"let\" would have been more appropriate. \"Leave,\" he says, is a slightly lower \"social register\" than \"let\" in that context. So much for Bo Peep. The two verbs are virtually interchangeable (in any social register) in the phrases \"Leave/let well enough alone\" and \"leave/let him/it be.\"
Authority: The New Fowler's Modern English Usage edited by R.W. Burchfield. Clarendon Press: Oxford, England. 1996. Used with the permission of Oxford University Press.
|