That vs. Which

Many people mistakenly use the words that and which interchangeably; however, they do not serve the same purpose and are not interchangeable. While some grammar experts complicate and obscure the rules dictating the usage of that and which, I strive to keep them as simple as possible! The simplest way to properly use them is to remember:

• That precedes an essential (or restrictive) clause
• Which comes before everything else.

That and the Essential Clause
To understand when to use that you must understand what makes a clause essential. An essential clause (also known as a restrictive or defining clause) provides a necessary part of the sentence by restricting or modifying another part of that sentence. Basically, you can’t get rid of the clause without losing part of the meaning and these clauses use that to introduce them. For example:

• Dogs that constantly whine annoy those around them.
  
  “That constantly whine” defines the type of dogs you are talking about. Without the clause you would be saying that all dogs annoy those around them, changing the meaning of the sentence. That comes with a clause that cannot be thrown out.

(Note: With essential clauses and therefore with that there are no commas used to separate the clause from the rest of the sentence.)

Which and the Non-Essential Clause
Which introduces the non-essential, additional information to the sentence. The non-essential clause (also known as the non-defining or non-restrictive clause) provides information in the sentence that can be removed without changing the meaning of the sentence. Which clauses can be thrown out and the sentence meaning remains the same. For example:

• Dogs, which sometimes bark out of excitement or fear, make good pets.
  
  “Which sometimes bark out of excitement or fear” can be removed from the sentence and the integrity or meaning of the sentence does not change - “Dogs make good pets.”

(Note: commas are always used to separate the non-essential information from the rest of the sentence.)

Wrap It Up
That provides vital information that cannot be removed from the sentence without changing the meaning, and commas are not needed. Which introduces additional, non-essential information to a sentence, and commas are always necessary.

Bonus Tip
Remember: Do not use which or that to refer to a person. Use who (or whom or whose). See Tips and Tricks entry on Who vs. Whom.