Writers, even experienced ones, frequently make some simple mistakes. Once writers develop correct usage into habits, they can avoid these mistakes.
The following words appear in texts frequently and are often spelled incorrectly. This list does not include locale-specific differences in spelling.
INCORRECT: dependant, dependancies CORRECT: dependent, dependencies INCORRECT: kernal CORRECT: kernel INCORRECT: seperate CORRECT: separate
A v. an -- The choice of which article to use, 'a' or 'an', is made by the sound of the first letter of the word following the article. The 'a' is used when the initial sound is a consonant, the 'an' is used when the initial sound is a vowel. The word 'SSH' starts with the "ess" sound, so the 'an' is used.
One common confusion is with acronyms and abbreviations, because that can change how you pronounce something. For example, if 'URL' is pronounced "You arr ell," it is "a URL". If 'URL' is pronounced "Earl" ("Urrl"), it is "an URL."
References: http://www.drgrammar.org/faqs/#36 and http://grammar.ccc.commnet.edu/GRAMMAR/abbreviations.htm
Sentences using "which" or "that" to separate statements frequently contain mistakes in the usage of commas. An instance of "which" is preceded by a comma, which is often forgotten. Instances of "that" are often incorrectly preceded by a comma that should not be present. When in doubt, trust your ear. "Which" wants a pause before it and "that" does not.
Use a comma for serial lists before the last item, that is:
"Foo, bar, and baz."
... not ...
"Foo, bar and baz."
Without the final comma, it is hard to tell if the final item is a "bar" or a "bar and baz". (Writers and editors sometimes refer to this punctuation as a "Harvard comma" or "serial comma.")
Avoid contractions in professional writing. They are rarely needed or desired, and they are a common source of errors. When using them, writers often make errors that include spelling, placement of apostrophes, and improper usage within a sentence. The first rule to remember about contractions is that the apostrophe should be placed where the last omission of letters occurs.