English grammar has a reputation for being confusing, but most of the common mistakes come down to a handful of mix-ups. Once you know the trick behind each one, they stop being a problem.
Your vs. You're
Your shows possession ("your book"). You're is short for "you are" ("you're late"). Trick: if you can replace it with "you are" and the sentence still makes sense, use "you're."
Its vs. It's
Its shows possession ("the dog wagged its tail"). It's is short for "it is" or "it has." Same trick: if "it is" fits, use the apostrophe.
There, Their, and They're
There refers to a place ("put it there"). Their shows possession ("their house"). They're is short for "they are" ("they're coming").
Less vs. Fewer
Use fewer for things you can count ("fewer apples"). Use less for things you can't count individually ("less water"). If you can put a number in front of it, use "fewer."
Who vs. Whom
Who is the subject of a sentence ("who called you?"). Whom is the object ("to whom it may concern"). Trick: if you could answer with "he/she," use "who." If you'd answer with "him/her," use "whom."
i.e. vs. e.g.
i.e. means "that is" — it clarifies. e.g. means "for example" — it lists examples. "I like citrus fruit, e.g. oranges and lemons" vs. "I like citrus fruit, i.e. fruit high in vitamin C."
Affect vs. Effect
Affect is almost always a verb ("the weather affects my mood"). Effect is almost always a noun ("the effect was obvious"). Trick: Affect = Action, Effect = End result.
None of these mistakes make you sound less intelligent — even native speakers mix them up constantly. But getting them right consistently is exactly the kind of polish that a structured course with real grammar explanations and native-speaker feedback can help lock in.