Grammar mistakes rarely come from not knowing the rule — they come from writing faster than you can proofread. The errors below are the ones that slip past spell-checkers and second readings alike, so it helps to know exactly what to look for.
Its versus it's
It's is always a contraction of "it is" or "it has." Its is the possessive, with no apostrophe, in the same family as "his" and "her." A quick test: if you can replace the word with "it is" and the sentence still makes sense, you need the apostrophe. The company changed its policy is correct; the company changed it's policy is not.
Dangling modifiers
A modifier needs something to attach to. Walking to the office, the rain started falling technically says the rain was walking. Fix it by giving the modifier a proper subject: Walking to the office, I got caught in the rain. Read the opening phrase and ask who or what it describes — if the answer isn't the sentence's actual subject, rearrange it.
Subject-verb agreement and comma splices
Words like "team," "data," and "each" trip people up because they feel plural but often take a singular verb. Each of the reports is due Friday, not "are due," because "each" is the true subject, not "reports." A related habit is the comma splice — joining two complete sentences with only a comma, as in She finished the draft, she sent it immediately. Fix it with a period, a semicolon, or a conjunction: She finished the draft, and she sent it immediately.
None of these mistakes are signs of bad writing; they're signs of writing quickly, which everyone does. Build one final proofreading pass into your process that hunts for exactly these patterns, and most of them disappear before anyone else sees the draft. Read the piece once purely for grammar, separately from editing for sense, and you'll catch far more than trying to do both at once.
