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Which or that?

Both which and that are used in the same way to introduce subordinate clauses, but they are not interchangeable; the choice depends on whether the clause is restrictive (also called defining) or non-restrictive, which is also indicated by punctuation. Here are some examples:

A1. My car, which is blue, needs painting.

A2. My car, that is blue, needs painting.

A3. My car which is blue needs painting.

A4. My car that is blue needs painting.

Sentence A1 would be used if I own only one car and it needs painting. The fact that it is blue is incidental. I am only adding that in as additional information: I could leave it out and the sentence would still make sense. This is a non-restrictive clause; it must be set off by commas, and it must use the word which (or who, for a person).

Sentence A2 would have been correct English about 200 years ago, but today it is simply wrong. That, used in this way, can only introduce a restrictive clause, and a restrictive clause must not be set off by commas. Note that this is different from German, for example, where commas are required around restrictive as well as non-restrictive clauses. It is a common error made by Germans writing in English to use A2 when they mean A4.

On the other hand, I would use sentence A3 or A4 if I own more than one car and am talking about a particular one of them, namely the blue one. Now the clause "that is blue" or "which is blue" is essential to the sentence: without it, the reader would not know which of my cars I meant. This is a restrictive clause and must not be set off by commas.

Some style books recommend that just as non-restrictive clauses always use which, so restrictive clauses should always use that, and some people actually consider it an error to do otherwise. Thus the usage in A3 is disputed; some people consider it wrong or at least inferior, but many others accept it. A writer who wants to avoid disputed usage will conform to the guideline in any case, and use A4 in preference to A3. That is:

• When the clause is not essential to the sentence (is non-restrictive), use which, and set off the clause with commas.
• When it is essential (restrictive), use that, and do not use commas (unless they are needed for some other reason).

Here is another example:

B1. The studies, which were written by students, were well researched.

The clause is non-restrictive. All of the studies were well researched, and incidentally, all of them were written by students.

B2. The studies, that were written by students, were well researched.

This version is wrong.

B3. The studies which were written by students were well researched.

The clause is restrictive: only some of the studies were written by students, and those are the well-researched ones. The usage of which is disputed.

B4. The studies that were written by students were well researched.

Same meaning, but acceptable to everyone.

Its or it's?

It's a simple rule to distinguish its and it's:

• Its (without an apostrophe) is always the genitive, the possessive case which is an equivalent to "his", "your", or "Michael's."
• It's (with an apostrophe) is the contraction of it is or it has, which is why this question is complicated for many people.

Many people find this distinction difficult because they are used to thinking that that if it belongs to someone, then it must take "apostrophe-s". This is incorrect. When it is a genitive pronoun (which relates to a specific person or thing), there is never an "apostrophe-s". There is no apostrophe in my or your or theirs, and there should not be one with its.

The simple rule for determining whether its/it's should be used is:

use it's only if the sentence would make sense if it is or it has were substituted.

For example: It's cloudy outside.

• "It is cloudy outside" makes sense, so it's is correct.

It's been a while since we met.

• "It has been a while since we met" makes sense, so it's is also correct here.

The dog wagged its tail.

• "The dog wagged it is tail" is grammatically incorrect, and does not make sense, so its should be used.

The Misplaced Pages:Manual of Style (Section 5.4) says, "In general, formal writing is preferred. Therefore, avoid excessive use of contractions — such as "don't, can't, won't, would've, they'd", and so on — unless they occur in a quotation." Contractions such as it's should therefore usually be expanded when used on Misplaced Pages.

Ones or one's?

Contrary to the above rule for its, the genitive of the pronoun one always uses an apostrophe.

For example: One should always wash one's feet in the morning.

The only situation in which ones can be used is as the plural of the number 1.

For example: The number 6411 contains two ones.

There, their or they're?

These are, due to being same-sounding, often confused; they are to be used as follows:

• There refers to a specific location or introduces a clause or sentence

My blue car is over there.

There is no faster car than that one.

• Their is a possessive pronoun, meaning "belonging to them"

Their car broke down, so they had to call a technician.

• They're is simply a contraction of "they are"

They're waiting for you.

A combination of the three can be compiled:

Their house is over there; they're not home right now, though.

A mnemonic phrase for the three homonyms is “they're there in their room”.

Your or you're?

As above, these are same-sounding but not interchangeable:

• Your is a possessive pronoun, meaning "belonging to you"

Your house is nicely decorated.

• You're is simply a contraction of "you are"

You're a commendable designer.

Once again, a combination of these could be made:

Your plane is delayed yet you're very calm.

Could or couldn't care less?

This is an issue to which logic is the answer:

• I could care less means "I do care somewhat", since one can only care less if already caring
• I couldn't care less means "I don't care at all", since one cannot care less if not caring

Evidently, if you mean to say "I don't care at all", the latter term, I couldn't care less, is the one to use. The matter changes slightly in American English, where both colloquialisms are used interchangably to mean "I don't care at all", with "I could care less" meaning "I could hardly care less"..

Than or then?

• Than is to be used when making comparisons

You know better than to do that.

• Then specifies a point in time or introduces the next in time, space or order

Dwight Eisenhower, then the president of the United States, signed defence treaties with South Korea and the Republic of China.

I wasn't even born then.

He did his speech, then the curtains closed.

Practice vs. practise

Practice is a noun.
Practise is a verb. (except in the United States)

Using practice as a verb is a common mistake: it is always a noun, except in the United States. Practicing and practiced are therefore spelling errors in non-US English. The correct spellings are practising and practised.

Bill needed more practice in swinging his golf club, so he went to the golf course to practise.

The following pairs of words work in the same way, i.e. the noun is spelled with a "c", and the verb is spelled with a "s":

• licence/license (except in the United States, where license is used for both the noun and verb)
• advice/advise
• device/devise

In the United States

In the United States, practice is both the verb and the noun. Practise is not commonly used:

Uncle Sam needed more practice in swinging his golf club, so he went to the golf course to practice.

See also

• List of frequently misused English words
• List of English words with disputed usage
• Misplaced Pages:Common grammatical errors

Useful resources

• Get It Write
• Dictionary of Same-Sounding Words