Good grammar is important. The fundamental purpose of grammar is to ensure that the
meaning the author intends to convey is successfully transmitted to the reader. (Hence the humor in "
Eats, shoots, and leaves").
But
perfect grammar is not necessarily the objective. The
New York Times has a column specifically devoted to the niceties of grammar and usage, and not infrequent errors, in recent
Times articles. For example:
Can you spot the error in that? For the purists:
You could spend an awful lot of time on a quest for perfection, without really adding much to either your understanding of the issue at hand, or your effectiveness as a communicator.
Yes: Strive to keep to the basic rules of English grammar. Agreements in case, between subject and verb. Pay some attention to the hazards of homophones ("their"; "they're"; "there"). I cannot tell you how many times calling someone a "looser" has caused private guffaws among readers.
But don't obsess over it.
Personally, I think a short volume on the general principles of short essay writing, such as the classic
The Lively Art of Writing; with its emphasis on the structure and composition of effective and enjoyable writing may be a good investment of the student's time.