earth, Earth
Generally lowercase; capitalize when used as the proper name of the planet. “What on earth are you talking about?” The astronauts returned to Earth.  

See affect, effect.

Meaning for example, it is always followed by a comma.

Use it to mean one or the other, not both. Correct: She said to use either door. Incorrect: There were lions on either side of the door. Correct: There were lions on each side of the door. There were lions on both sides of the door.

either…or, neither…nor
The nouns that follow these words do not constitute a compound subject; they are alternate subjects and require a verb that agrees with the nearer subject: Neither they nor he is going. Neither he nor they are going.

Always hyphenate and lowercase: President-elect Carter.

Avoid using as a verb: He emailed her the instructions. Preferred: He sent the instructions to her in an email message; He sent the instructions to her via email. Also, see computer terminology.

emeritus, emerita, emeriti
This word often is added to formal titles to denote that individuals who have retired retain their rank or title. When used place "emeritus" after the formal title, in keeping with the general practice of academic institutions: Professor Emeritus of Religion Robert Albertson, Professor Emerita of Comparative Sociology Ann Neel.

emigrate, immigrate
One who leaves a country emigrates from it. One who comes into a country immigrates. The same principle holds for emigrant and immigrant.

ensure, insure
Use "ensure" to mean guarantee: Steps were taken to ensure accuracy. Use "insure" for references to insurance: The policy insures his life.

Means the right to do or have something. Do not use to denote the name of a book or speech: She was entitled to the promotion. The book was titled Gone With the Wind.

every day (adv.), everyday (adj.)
She goes to work every day. He wears everyday shoes.

See accept, except.

exclamation point
The exclamation point is used sparingly in good writing. Two or three exclamation points in a row should be reserved for an invasion of Martians.