insure and ensure

Insure means to indemnify, e.g. compensate for loss, ensure means to make certain, see to it that the loss doesn't happen.

Less and Fewer

Fewer means the quantity is ascertainable, i.e. can be counted, while less means, simply, that it is of smaller magnitude, though the precise quantity is not discernible, or, perhaps, even relevant. One might say, "less current," or one might say, "fewer milliamperes."

RE