"Every Day" and "Everyday"
The adverbial phrase "every day" is often misspelled into the word "everyday," which is a word with a different meaning. "Everyday" is an adjective and is used to describe something that is common, mundane, occurring every day, usual or ordinary.

"Every day," on the other hand, is an adverbial phrase and a time expression. Nowadays, the big problem is when people turn "every day" into "everyday." For example, an incorrect usage of the word "everyday" would be "I play video games everyday." The correct way to write the sentence should be "I play video games every day." The word "everyday" should be used as in, "One of my everyday duties is to clean my bedroom."

"Whose" and "Who's"
The possessive form of "who," "whose," is often misspelled into "who's." For example, it is incorrect to write "Who's fault is it?" It should be "Whose fault is it?"

"Who's" always means either "who is" or "who has." (Examples: "Who's the person wearing the pink hat?" and "Who's been playing cards with him?")

"All Right" and "Alright"
"Alright" has been so popular that perhaps many people do not notice that it should actually always be two separate words. But if you want to stay on the safe side and avoid irritating others, you’d better write "all right" instead of "alright."

"Every Time" and "Everytime"
"Every time" is often misspelled into "everytime," which is not a word. The phrase "every time" is always two separate words. Do not write it as a single word.

"Never Mind" and "Nevermind"
"Never mind" should always be two separate words. When the word means "attention," like in the expression "pay him no nevermind," it is said to be one word. However, "nevermind" isn't a word, and its usage as "attention" is slangy. Do not write "never mind" as a single word. It is two separate words.

"Shone" and "Shined"
When "shine" is a verb that is used without an object, its past tense and past participle is "shone." However, when "shine" is a verb that is used with an object, its past tense and past participle is "shined."

For example, in "After I had shined my shoe with water, it shone brightly," where "shined" means "polished" and "shone" means "sparkled," "shined" is used with an object (my shoe) and "shone" is used without an object.

"Hung" and "Hanged"
When "hang" is a verb that is used for objects and items (e.g. a painting, a poster), its past tense and past participle should be "hung." However, when "hang" is a verb that is used when executing or suspending someone, its past tense and past participle should be "hanged," as in "the criminal was hanged."

"Media" and "Mediums"
When "medium" is referring to a person who receives and transmits messages from the spirits of the dead, as in "spirit medium," its plural is "mediums." However, when "medium" means something other than a spirit medium, its plural is always "media" (e.g. "newspapers and television are good media for advertising," "the artist prefers using traditional media, such as crayons and markers, for her works," "sound waves travel at different speeds in different media").

Like "criterion" and "phenomenon" (plural: "criteria" and "phenomena"), "medium" is a word whose plural form ends in A. The only exception comes when you are referring to a spirit medium.

"Affect" and Effect"
"Affect" and "effect" are each both nouns and verbs. "Affect" is usually a verb that can mean "have an influence on," "make use of," "touch the heart of," etc. It is accented on the final syllable.
However, when relating to emotions and feelings (or psychiatry, social scientists, etc.), "affect" as a noun is present and is accented on the first syllable.

"Effect" is usually a noun that can mean "a result (of a cause or an action)," "a result produced upon the mind," or "force or meaning (of a clause)," etc.
However, as a verb, it is used to mean "to bring about" or "to create," as in "I have decided that I should effect a change in tomorrow's plan."

Please note that one's personal "effects" is one's personal property.

"Lay" and "Lie"
"To lay " and "to lie" are not the same. When you go to bed, you lie down, but when you act upon another object, you lay the object, as in "to lay down a book." In the present tense, when the subject is lying down, the word for it is "lie." If the subject is acting upon another object, the word for it is "lay."

The past tense of "lie" is "lay," and its past participle is "lain."
Past tense: "I lay down."
Past participle: "I have lain down a lot during the summer."

However, the past tense and past participle of "lay" is both "laid."
Past tense: "He laid down the ticket."
Past participle: "I have laid down my camera."

Please note that you only add "–ing" to verbs in the present tense. It is incorrect to write "I was laying down yesterday.” It should be "I was lying down yesterday.”

Another thing you should take note of is that verbs followed by "to" should always be in the present tense. It is incorrect to write “I wanted to lay down yesterday.” It should be "I wanted to lie down yesterday.”

"e.g." and "i.e."
The abbreviation (e.g.) for the Latin phrase exempli gratia should be used when you mean "for example." However, when you mean "that is," use i.e., the abbreviation of the Latin phrase id est.

Please note that since "e.g." is used to indicate a partial list, adding "etc." at the end of a list that is introduced by this abbreviation is redundant.

"Accidentally? Accidently? Huh?"
Sometimes people have trouble spelling "-ly adverbs" (e.g. "accidentally," "unfortunately," "definitely"). However, please remember that, most of the time, these -ly adverbs are formed from adjectives, by adding "-ly" to them.

For example, "accidentally" comes from the adjective "accidental."
"accidental (adjective)" + "-ly" = "accidentally (adverb)"

"Definitely" comes from the adjective "definite."
"definite (adjective)" + "-ly" = "definitely (adverb)"

When the adjective ends in a "y," as in "lucky," you replace the "y" with an "i" before you add the "-ly."
"lucky (adjective)" + "-ly" = "luckily (adverb)"

Please remember, this is just an effective way to spell many of the "-ly adverbs." There are still some exceptions (e.g. "truly," "basically").

Please note that not all words that end in "-ly" are adverbs (e.g. "friendly"), nor do adverbs always end in "-ly" (e.g. "never," "everywhere").

"Had Better" and "Better"
When you hear someone say "I better wash the dishes before my mom gets home," they actually mean "I'd better" (abbreviation of "I had better"), in which you usually don't hear the 'd.

"Better," the comparative of "good," should never be written without "had" in this sense.