advice and advise
Advice is a noun meaning an opinion.

She gave good advice.

Advise is a verb meaning to give an opinion or recommendation.

He advised her to own up.

affect and effect
Affect is usually a verb and means to influence.

The weather affected the outcome of the Grand Prix.

Effect is usually a noun and means the result.

The effect was dramatic.

Effect can occasionally be a verb meaning to produce a result (to effect a change) and affect can occasionally be a noun when describing emotion (sad affect).

Tip: Remember ‘a’ for action.

complement and compliment
Complement as a verb means to go with and complete. Remember this by thinking that things that complement each other are often complete. Complement as a noun is the quantity or amount that completes something.

Pepper and salt complement each other. (verb)
We have a full complement here today. (noun)

Compliment is a noun meaning a flattering remark and a verb meaning to make a flattering remark. It’s also a formal expression of civility or respect (e.g. compliments of the season).

He complimented her on her appearance. (verb)
She received a compliment about her appearance. (noun)

If you give someone a bonus to go with a product, you can use either complimentary or complementary.

dependant and dependent
In Australian and New Zealand spelling, dependant is a noun and means someone who depends on support from others. This support often has a financial component.

The child is a dependant.

Dependent is an adjective and means relying on.

The outcome is dependent on the test results.

American spelling uses dependent for both meanings. Much simpler!

desert and dessert

A desert is a noun meaning a wasteland, while the verb desert means to abandon. Deserts is also used in the expression, ‘get your just deserts’, i.e. what you deserve.

The explorers were lost in the desert for four days without water. (noun)
He deserted his post when the gunfire started. (verb)

Dessert is a noun meaning puddings or a sweet dish at the end of a meal.

We had pavlova and ice cream for dessert on Christmas day.

its and it’s
It’s only has an apostrophe when it is short for it is or it has.

It’s a lovely day. (It is a lovely day.)
It’s been a lovely day. (It has been a lovely day.)

Its without an apostrophe indicates possession or ownership.

The airline has drastically cut its prices. (prices belong to the airline)

lose and loose
Loose is an adjective meaning the opposite of tight.

My trousers are loose because I’ve been on a diet.

Lose is a verb meaning to come to be without, be defeated, or unable to find something.

I may lose my mind.
We may lose the game.

principal and principle
Principal means leader, first in rank, and an original investment of money. It can be used as a noun or adjective.

The principal of the school gave the speech. (Think of ‘your principal is your pal’ for this noun.)
The principal conductor of the orchestra led the rehearsal. (adjective)

Principle is a noun meaning values, fundamental truth or law of nature.

One of the basic principles of democracy is freedom of speech.

stationary and stationery
Stationary is an adjective meaning standing still, immobile.

The car was stationary.

Stationery is a noun referring to writing materials.

I ordered the stationery.

Tip: Remember ‘e’ for ‘envelopes’.

their, there and they’re
Their is a possessive pronoun that modifies a noun.

They ate their dinner.

There is primarily used as an adverb relating to place, but can also be used as an adjective (that there man) and a pronoun (There is no hope).

I flew there in a jet plane.

They’re is an abbreviation of they are (pronoun and verb).

They’re late for the meeting.