From Kenneth Beare,
Your Guide to English as 2nd Language.

English Commonly Confused Word Pairs for ESL Learners

Part I
Here are some of the most commonly confused English word pairs. They have been chosen especially for ESL learners. The list is not complete, if you have any commonly confused words that you feel should be included. Send me an esl.guide@about.com.
beside / besides

beside: preposition meaning 'next to', 'at the side of'

Examples:

I sit beside John in class.
Could you get me that book? It's beside the lamp.

besides: adverb meaning 'also', 'as well'; preposition meaning 'in addition to'

Examples:

(adverb) Besides tennis, I play soccer and basketball.
(preposition) That is a beautiful lamp beside the table.

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clothes / cloths

clothes: something you wear - jeans, shirts, blouses, etc.

Examples:

Just a moment, let me change my clothes.
Tommy, get your clothes on!

cloths: pieces of material used for cleaning or other purposes.

Examples:

There are some cloths in the closet.

Use those to clean the kitchen.
I have a few pieces of cloth that I use.
dead / died

dead: adjective meaning 'not alive'

Examples:

Unfortunately, our dog has been dead for a few months.
Don't touch that bird. It's dead.

died: past tense and past participle of the verb 'to die'

Examples:

His grandfather died two years ago.
A number of people have died in the accident.

experience / experiment

experience: noun meaning something that a person lives through, i.e. something that someone experiences. - also used as an uncountable noun meaning 'knowledge gained by doing something'

Examples:

(first meaning)His experiences in Germany were rather depressing.
(second meaning) I'm afraid I don't have much sales experience.

experiment: noun meaning something that you do to see the result. Often used when speaking about scientists and their studies.

Examples:

They did a number of experiments last week.
Don't worry it's just an experiment. I'm not going to keep my beard.

felt / fell

felt: past tense and past participle of the verb 'to feel'

Examples:

I felt better after I had a good dinner.
He hasn't felt this well for a long time.

fell: past tense of the verb 'to fall'

Examples:

He fell from a tree and broke his leg.
Unfortunately, I fell down and hurt myself.

female / feminine

female: the sex of a woman or animal

Examples:

The female of the species is very aggressive.
The question 'female or male' means 'are you a woman or a man'.

feminine: adjective describing a quality or type of behaviour that is considered typical for a woman

Examples:

He's an excellent boss with a feminine intuition.
The house was decorated in a very feminine manner.

its / it's

its: possessive determiner similar to 'my' or 'your'

Examples:

Its color is red.
The dog didn't eat all of its food.

it's: Short form of 'it is' or 'it has'

Examples:

(it is) It's difficult to understand him.
(it has) It's been a long time since I had a beer.