A lot, Alot - Alot is not a word.

Accept - To Receive or believe.
Except - Other than.

Affect - To influence something.
Effect - The result of something.

Beside - Nearby.
Besides - Other than or in addition to.

Elicit - To bring out.
Illicit - Unlawful.

Eminent - Prominent, conspicuous, or famous.
Imminent - Threatening to occur immediately; near at hand; impending.

Healthy -Possessing good health.
Healthful -Conducive to good health.
   See Note Below

Immigrate -To come into a new country
Emigrate -To leave one country and live in another one.

Less -Used with mass terms for things of measurable extent.
Fewer -Used for things that can be counted.
   See Note Below

Principal -Primary. A school administrator. A sum of money.
Principle -An idea or doctrine.

Stationary - Not movable; fixed.
Stationery - Paper and envelopes specifically for writing letters.

Than - Used in comparisons.
Then - Used to tell when.
   More Info

I hope that clears a few things up.
From the American Heritage Dictionary:

Healthy vs. Healthful
The distinction in meaning between healthy ("possessing good health") and healthful ("conducive to good health") was ascribed to the two terms only as late as the 1880s. This distinction, though tenaciously supported by some critics, is belied by citational evidence—healthy has been used to mean "healthful" since the 16th century. Use of healthy in this sense is to be found in the works of many distinguished writers, with this example from John Locke being typical: "Gardening . . . and working in wood, are fit and healthy recreations for a man of study or business." Therefore, both healthy and healthful are correct in these contexts: a healthy climate, a healthful climate; a healthful diet, a healthy diet.

Less vs. Fewer
The traditional rule holds that fewer should be used for things that can be counted (fewer than four players), while less should be used with mass terms for things of measurable extent (less paper; less than a gallon of paint). However, less is used in some constructions where fewer would occur if the traditional rule were being followed. Less than can be used before a plural noun that denotes a measure of time, amount, or distance: less than three weeks; less than $400; less than 50 miles. Less is sometimes used with plural nouns in the expressions no less than (as in No less than 30 of his colleagues signed the letter) and or less (as in Give your reasons in 25 words or less).

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