Affect / Effect

Affect as a verb means to influence, act upon, or change something.
Example: The noise inside the stadium affected his performance.

Effect is usually a noun (thing) meaning to have an impact on something or someone.
His smile had a strange effect on me.
Effect can also mean "the end result."

Example: The drug has many adverse side effects.
Verb. To influence
Noun. The resulting emotion
Verb. To cause to happen
Example: We were all affected by the drought.
The effect of the drought was an increase in food prices.
The drought effected an increase in price.

Amount / Number

Amount is a noun referring to non count nouns
ExampL: Unfortunately, a huge amount of oil leaked out of the tank.

Number is a noun referring to count nouns
Example: Though few in number, judges have much power in Greece.

Complimentary / Complementary

Complimentary is an adjective which means given freely, or giving praise
Example: The teacher was very complimentary about my work.

Complementary is an adjective, which means, supplying needs
Example : The complementary relationship of the bee and the flower is quite remarkable.

Different from / Different than

Different from is a preposition, which precedes a noun phrase
Ex: He is different from your average shop owner.

Different than is a preposition which precedes a noun clause
Ex: London isdifferent than we had imagined.

Enough

Enough is an adverb, which precedes a noun and follows an adjective

He knows enough English to study in England.
She is fast enough to win the race.

Every so often / Ever so often

Every so often is an adverb meaning 'occasionally'
Ex: They come every so often. I wish they could come more frequently.

Ever so often is an adverb meaning 'frequently'
Ex: They come ever so often. I wish they would stay home.

First / Former

First as an adjective refers to three or more items
Ex: The first five skiers fell.

Former as an adjective refers to two or fewer items
Ex: The former Secretary of State for the U.S., Colin Powel, was the first black to hold that position.

From / Since / For

From is a preposition followed by a noun or noun phrase.
Ex: As a time marker, it requires to or another preposition.
From now on I will lead the way.
From Monday to Friday, I work like a slave.

Since is a subordinate conjunction followed by a clause.
It expresses the time something began or 'the beginning of time'
Since Wednesday, I have walked 6 miles a day.

For is a preposition followed by a noun or noun phrase.
It expresses ?the length of time (duration) something has lasted.
For two weeks I have walked to work every day.

Lie / Lay

Lie is an intransitive verb meaning 'to recline'
He lies down for a nap after his lunch.

Lay is a transitive verb which means 'to put or place'
He lay the book on the desk and left the room.

A Few / A little

A few means 'some / not many.' Used with countable nouns.
Ex: A few people were standing outside the shop waiting to get in.

A littlemeans 'some / not much.' Used with non-count nouns.
Ex: He gave me a little cheese to eat with my bread.

Passed / Past

Passed is a transitive verb and past participle of the verb pass
Ex: She barely passed the exam.

Past is a preposition or adjective meaning 'by'
Ex: We will keep school open past June.
Ex: She walked past without saying hello.

Raise / Rise

Raise is a transitive verb meaning to move to a higher place.
Ex: Tom raised his hand to answer a question.

Rise is an intransitive verb meaning to go up or ascend.
Ex: The sunrises in the morning.

A lot or Alot

A lot should be written as two words. Although a lot is used informally to mean "a large number" or "many," avoid using a lot in formal writing. Example: "The crook had many (not a lot of) chances to rob the stranger."

Accept or Except

Accept is a verb meaning 'to receive' or 'to approve.'
Example: 'I accept your offer of the book.'

Except is a verb meaning 'to leave out' or 'to exclude.'
Example: He excepted all Corvettes from his list of favorite cars.
Except can also be a preposition meaning excluding or leaving out.
Example: He liked everything on the plate except the liver.

Acronyms and initialisms

According to The Business Writer's Handbook, "An acronym is an abbreviation that is formed by combining the first letter or letters of several words. Acronyms are pronounced as words and are written without periods.
Ex: Radio detecting and ranging/radar Common Business-Oriented Language/ COBOL self-contained underwater breathing apparatus/ scuba

An initialism is an abbreviation that is formed by combining the initial letter of each word in a multiword term. Initialisms are pronounced as separate letters.
Ex: end of month/ e.o.m. cash on delivery/ c.o.d. post meridian/ p.m.
Usage guidelines:

The following are sample guidelines to apply in deciding whether to use acronyms and initialisms:
1. If you must use a multiword term as much as once each paragraph, you should instead use its acronym or initialism. For example, a phrase such as "primary software overlay area" can become tiresome if repeated again and again in one piece of writing; it would be better, therefore, to use PSOA.

2. If something is better known by its acronym or initialism than by its formal term, you should use the abbreviated form. The initialism a.m., for example, is much more common than the formal ante meridiem. If these conditions do not exist, however, always spell out the full term.

3. The first time an acronym or initialism appears in a written work, write the complete term, followed by the abbreviated form in parentheses.

Ex: "The Capital Appropriations Request (CAR) controls the spending of money." Thereafter, you may use the acronym or initialism alone. In a long document, however, you will help the reader greatly by repeating the full term in parentheses at regular intervals so that he or she does not have to search back to the first time the acronym or initialism was used to find its meaning.

4. Write acronyms in capital letters without periods. The only exceptions are those acronyms that have become accepted as common nouns, which are written in lowercase letters.

Ex.: "NASA," "HUD," "laser," "scuba." Initialisms may be written either uppercase or lowercase. Generally, do not use periods when they are uppercase, but use periods when they are lowercase. Two exceptions are geographic names and academic degrees.