As an editor, there are some common mistakes that I see in the work of most beginning writers. Some people say these mistakes are no big deal. They say that they prefer to write “the way they speak.”

The problem is that without the benefit of inflection and facial expressions, readers are at a disadvantage. So while conversational style writing is the preferred style these days, it’s still important to pay attention to grammar. Otherwise, no one will understand your message.

Grammar rules were created for a good reason – to ensure that your writing is clear. It doesn’t matter if your readers don’t know what’s wrong with your grammar. Even if they’re unable to pinpoint the errors, they’ll walk away distrusting your words if those words are poorly written. For some reason they can’t explain, they just won’t believe you.

So grammar isn’t about snooty librarians who want to keep everything pure for some Victorian ideal. It’s about your readers walking away with a clear understanding of your writing.

Here are the first 10 of the top 20 writing mistakes I see most often:

Sentences that go on for three or more lines – Rarely does it make sense to keep a sentence running for this length, especially if you have several clauses in it. By the time your readers reach the end of the sentence, they’ll no longer know what you’re talking about. It’s best to divide long sentences into two or three separate ones.

Changing tenses – This is one of the most common mistakes. You start out writing a paragraph in one tense and end it in another. For example: “He walks slowly toward the police station, worrying about his fate. When he got to the door, he took a deep breath before opening it.” The first sentence is written in the present tense, and the second is in past tense. It’s important to get this right because it’s very confusing and jarring for a reader.

Fear of contractions – This is an instance where writers sometimes try too hard. For most of today’s writing in articles, books, and online, it’s perfectly acceptable to use contractions. Unless academic writing or a particular publisher’s style guide prohibits it, go ahead and “contract.” Your prose will sound stilted and robotic to modern readers if everything is “will not” or “it is.” Don’t use contractions in all cases, but don’t be afraid to use them when they sound right to you.

Use of forms of the verb “to be” – Try to avoid writing phrases like, “She was being emotional.” “Being” is a very weak verb that rarely tells the reader anything useful, whether in fiction or non-fiction. Try to find a more descriptive verb, even if it’s something simple like “feeling” or “acting.”

Overuse of the word “then” – Don’t use the word “then” unless it’s truly necessary. Often, it adds nothing to your meaning. Writers put it in sentences because it’s what they say when they speak, but in writing, it can muddy a sentence needlessly. For example, the word “then” does nothing for this sentence: “We were at the doctor’s office for hours, but then, we arrived at the concert on time.” This sentence works just as well without it.

Misuse of “that” and “who” – Use “who” when referring to a human being. For example, you wouldn’t say, “Bob is the one that saw the accident.” You would say, “Bob is the one who saw the accident.”

“Could of” instead of “could have” – This is an easy mistake because when we say “could have,” it sounds like “could of.” While your average reader might understand what you mean, it’s wrong. If you think of the meaning of the word “of,” this sentence makes no sense: “Frank could of been there on time.” It’s correct to write: “Frank could have been there on time.”

“Times where” as opposed to “times when” – While people have begun to say, “There are times where” in casual conversation, the fact remains that time isn’t a place. It’s correct to say, “There are times when…”

“Its” versus “it’s” – This mistake has been written about countless times, but many writers still can’t remember how it works. The reason is that apostrophes often turn a word into a possessive, but that isn’t the case here. So how can you remember which is correct? “It’s” is the contraction for “it is.” When you write a sentence, try adding “it is” in place of “its” or “it’s.” If “it is” fits in your sentence, you need an apostrophe. If it doesn’t fit in your sentence, you don’t need the apostrophe. Simple as that!

“Their,” “there,” and “they’re” – This is another mistake that’s been written about a great deal but still confuses people. Here’s the difference between these words: “There” is a place, as in “We went there.” “Their” refers to something that belongs to people, as in “Their books.” “They’re” is the contraction for “they are,” as in “They’re in the house” (or “They are in the house.”)