As a whole, the English language makes almost no sense at all. Each “rule” typically has multiple exceptions, and pronunciation is highly inconsistent. A prime example of this is the fictional word “ghoti.” If you use the “gh” sound in “tough,” the “o” sound in “women,” and the “ti” sound from “nation,” “ghoti” is pronounced “fish.”

With this kind of linguistic wackiness, it’s very challenging to maintain proper spelling and grammar. However, poor language skills convey laziness, unprofessionalism, and the lack of intelligence. Bad grammar could prevent you from getting new opportunities, even if you’re exceptionally qualified. If you’ve mastered to/too/two and there/their/they’re, this guide will help you with some of the more difficult (but common) mistakes:

1. Would Have vs. Would Of
This common mistake can be attributed to a shift in casual spoken language. Though they’re not homophones, “have” can sound like “of” if spoken without proper pronunciation. A sentence like, “I would of gone had I known there would be free food,” is grammatically incorrect. Instead, you “would have gone.” If you get confused, ask yourself the sentence in question form. “What would you of done?” makes the error more obvious.

2. Couple vs. Couple Of
“I have a couple problems to discuss with you,” might sound normal when spoken aloud, but it’s never correct to omit the “of.” If you think about this one too much, it can get rather complicated. Just remember: you always have a couple of nouns.

3. A lot vs. Alot
This one is pretty straightforward. “Alot” isn’t a word, so you should always write it as “a lot.” You can remember this rule by telling yourself that you need (an) extra space for whatever it is you have a lot of.

4. Imply vs. Infer
Both of these words are verbs, but they’re often mistakenly used interchangeably. Writers or speakers imply in the words they use. A listener or reader infers something from the words. For example: “Ben implied that not all of the free food was gone. April inferred that there must have been more in the kitchen.”

5. Ensure vs. Assure vs. Insure
These three words all have similar pronunciations and indicate the making certain of an outcome, but they cannot be used interchangeably. To ensure is to make certain, to assure is to try to remove doubt from someone’s mind and to insure is to protect something financially with insurance. A traveler can insure her luggage, the gate agent can assure her that it will arrive at its proper destination and the airline can ensure that they fulfill that promise.