Respect your tools and your craft. These kinds of errors signal to your readers — including editors and other gatekeepers — that you don’t really know what you’re doing.
You should understand the following, and the problems they can cause:
(A writer needs to know what a sentence is!)
• run-on sentence (e.g. comma splice) vs. sentence fragment
• dependent vs. independent clauses
Homonyms and other easily confused terms
• their vs. there vs. they’re, your vs. you’re, then vs. than, affect vs. effect, two vs. too vs. to
• that vs. which, who vs. whom
Possessives and contractions
• Jays’ vs. James’s vs. James’
• its vs. it’s
• comma vs. semi-colon vs. colon
• parentheses vs. dash vs. hyphen
• punctuation for dialogue
Pronouns with unclear referents, and dangling modifiers
• e.g. “Jane told Sally that she was taking it too seriously.”
• e.g. “When it’s their turn to roll, the dice will be given to the player.”
• “I like to read, write and editing” and “Don’t be intimidated and afraid of the grammar terminology on this page” are e.g. of errors, while “It was both a long wedding and very beautiful” is inelegant, at the very least.
• A real-life example, published in the Perth Courier: “Modern agricultural practices such as pesticide use, habitat loss and climate change pose a serious threat to biodiversity.”
Grey areas, and prescriptivist vs. descriptivist grammar rules
• comma use (e.g. Oxford comma), hyphenating compound words
• split infinitives, and ending a sentence with a preposition
• ‘they’ as a generic singular genderless pronoun
• “begging the question” — traditional definition vs. current usage
How do you fix these problems?
Otherwise, grammar is one of the things the internet does right.
Yes, my professional editor’s opinion is that if there’s anything on this page you’re unsure of, ask the internet:
• Wikipedia, Grammar Girl, or the top hits on any websearch for grammar questions regularly outdo classics like the Chicago Manual of Style or the myth-propagating Strunk & White. Unlike these print-based grammatical authoritarians, they tend to be precise, concise, non-technical, and non-dogmatic.