The Importance of Grammar

Some people don't think that grammar is important. They are wrong. Grammar is important on a fundamental level. It wasn't established as a way to police how you write or to limit your creativity. It is there to help a reader gain clarity and understand the meaning of your words. This is obviously very important if you are trying to be a writer, but it is also good standard practice to have a good grounding in grammar for any job application or official correspondence you might have to write. I don't always get it right myself, so thought it would be good to do a post exploring common grammatical mistakes.

A few things to remember:

It's/Its:
"It's" is used as a contraction of the phrase "it is". The apostrophe represents the missing letter "i". "Its" (no apostrophe) means something belonging to something else. If in doubt, try replacing the word in your sentence with the phrase "it is" instead. Does it still make sense? No? Then don't use an apostrophe.

The semi-colon:
I can't explain it better than how it is here, and this has the added bonus of being humorous to boot.

There/Their/They're:
Basically, "There" is used to denote direction ("over there") or an abstract sense of place ("there once was a boy") (there is more to it than that, but I'll try to keep it simply - for further details see here.) Use "Their" if you mean "belonging to them"."They're" (it's that apostrophe again!) is a contraction of the phrase "they are".

That/Which:
A little bit of old school grammar here, but generally speaking, "that" is used if what you are referring to is important to the meaning of the sentence ("restrictive clause"), "which" is used if you could leave that clause off completely and it would still make sense ("non-restrictive clause"). There should never be a comma before "that", but always with "which".
I did a google search, and here are a couple of stolen examples from http://grammar.quickanddirtytips.com/which-versus-that.aspx:

Example 1: Gems that sparkle often elicit forgiveness
If you remove "that sparkle", it changes the meaning to say that all gems elicit forgiveness (and note there are no commas).

Example 2: Diamonds, which are expensive, often elicit forgiveness.
Diamonds are always assumed expensive, so if you remove the "which", the meaning remains.

For more on grammar, do have a look at http://grammar.quickanddirtytips.com/, which I found whilst researching this post. I'd also recommend reading The Elements of Style; a very handy guide.