Any RSVP to a formal invitation, whether declining or accepting, should use the same polite language that the invitation was written in. If the invitation was to "request the pleasure of your company," declining might be worded along the lines of "Mr. and Mrs. (invitee names) regret that they are unable to accept the kind invitation of..."
An explanation might be offered in this response or else relayed to the host in person.
If there is uncertainty about whether or not to attend, the host should be told about this. He should be informed as soon as the invitee knows for certain whether she will be able to attend.