I am often asked what sites I would recommend for online grammar guides. There is the standard Strunk’s Elements of Style. Another site to check is the Guide to Grammar and Style by Professor Jack Lynch. Professor Lynch is an associate professor at Rutgers. His guide is easy to read and provides detailed explanations. The site supports a basic search engine and an alphabetical listing of topics.
As an example, the guide’s entry for “assure, ensure, or insure” reads:
Ensure and insure are sometimes used interchangeably, but it may be better to keep them separate. Insuring is the business of an insurance company, i.e., setting aside resources in case of a loss. Ensure means make sure, as in "Ensure that this is done by Monday."