less, fewer The traditional view is that less is used for matters of degree, value, or amount, and that it modifies nouns that refer to uncountable things ("less concern," "less clothing") while fewer modifies numbers and plural nouns ("fewer students," "fewer than eight trees"). However, less is actually more likely than fewer to modify plural nouns when distances, sums of money, and certain common phrases are involved ("less than 100 miles," "less than $2,000," "ten pounds, more or less") and just as likely to modify periods of time ("in less [fewer] than four hours"). But phrases such as "less bills," "less vacation days," and "less computers" should be avoided. (来源：英语学习门户网站EnglishCN.com)

like, such as Though sometimes criticized, it is completely permissible to write "cities like Chicago and Des Moines." It is equally proper and slightly more formal to write "cities such as Chicago and Des Moines" or "such cities as Chicago and Des Moines."

loan Some people still object to the use of loan as a verb ("loaned me the book") and insist on lend. Nevertheless, loan is in standard use. Loan is used only literally ("loans large sums of money"), however, while lend can be used both literally ("lends large sums of money") and figuratively ("lending his parents some help when he could").

media Media is the plural of medium. In references to the mass media today, media is often used as a singular mass noun ("The media always wants a story"). But this singular use is not as common as the similar use of data, and you may want to keep media plural in your writing ("The news media have ignored the story").

myself Myself is often used in place of I or me: as subject ("Others and myself continued to express our concern"); after as, than, or like ("paying such people as myself to teach," "Other than myself, only two volunteers showed up," "A man like myself worries about these things"); and as object ("For both my wife and myself, it was a great trip"). It is often used when the person is referring to himself or herself as an object of discussion. Though these uses have been criticized, they are entirely standard.

one Using one in place of another personal pronoun (I, me, you) or to indicate a generic individual produces a formal effect ("not as helpful as one might wish," "This might strike one as a clever solution"). As a substitute for I or me, it is far more common in British English than American English.

people, persons People is commonly used when referring to an unspecified number of individuals ("People everywhere are talking about the new show"), and persons is commonly used when a definite number is specified ("reported that 86 persons were still missing"). However, the use of people where numbers are mentioned is also acceptable and much less formal ("The police questioned ten people").

per Per, meaning "for each," is most commonly used with figures, usually in relation to price ("$400 per performance"), vehicles ("25 miles per gallon," "55 miles per hour"), or sports ("15 points per game"). Avoid inserting words like a or each between per and the word or words it modifies, producing sentences like "She could type 70 words per each minute."

plus The use of plus to mean "and" ("a hamburger plus french fries for lunch") or "besides which" ("We would have been on time, but we lost the car keys. Plus, we forgot the map") is quite informal and is avoided in writing.

presently The use of presently to mean "at the present time" ("I'm presently writing a report") rather than "soon" ("He'll be with you presently"), while often criticized, is standard and acceptable.

pretty Pretty, when used as an adverb to tone down or moderate a statement ("pretty cold weather"), is avoided in formal writing, so using it in correspondence will lend an informal tone.

prior to Prior to, a synonym of before, most often appears in fairly formal contexts. It is especially useful in suggesting anticipation ("Prior to the meeting, everyone on the committee had studied the report").

proved, proven Both proved and proven are past participles of prove. Proved was once more common than proven, but today they are about equally common. As a past participle, either is acceptable ("a drug that has been proved [proven] effective"), but proven is more frequent as an adjective ("proven gas reserves").

providing, provided Although providing in the sense of "if" or "on condition that" has occasionally been disapproved, both providing and provided are in common and accepted use ("providing [provided] he finds a buyer").

real The adverb real is used interchangeably with really only as an intensifier ("a real tough job"). This use is very common in speech and casual writing, but you should not use it in anything more formal.

set, sit Set generally takes an object ("Set the lamp over there") and sit does not ("sat for an hour in the doctor's office"). But in a few cases, set lacks an object ("The sun will set soon") and sit takes an object ("She sat the child down by its grandfather").

shall, will Shall and will are generally interchangeable today in American English, but shall is often regarded as somewhat affected, and will is much more common. However, shall is more appropriate in questions to express simple choice ("Shall we go now?"), because will in such a context suggests prediction ("Will the report be ready next week?").

since The conjunction since, when used to mean "because," can occasionally create ambiguity when both this "because" sense and the alternative "from the time when" sense are meaningful in the same context ("Since the cold weather had started, he had been thinking about the expense of heating his house"). But there usually is no ambiguity ("Since it was raining, she took an umbrella") and thus no reason to avoid it.

slow, slowly Slow used as an adverb (meaning "slowly") has often been called an error, though it has been common for centuries. Slow is almost always used with verbs indicating motion or action, and it typically follows the verb it modifies ("stew should be cooked long and slow"). Slowly can be used in the same way ("drove slowly"), but it also is used before the verb ("The winds slowly subsided"), with adjectives formed from verbs ("the slowly sinking sun"), and in places where slow would sound odd ("turned slowly around").

so The use of the adverb so to mean "very" or "extremely" is rare in formal writing, except in negative contexts ("not so long ago") or when followed by an explaining clause ("coffee so hot that I burned my tongue"). The use of the conjunction so to introduce clauses of result ("The weather was beautiful, so we decided to go for a walk") is standard. When it introduces clauses of purpose, with the meaning "in order that" ("Be quiet so I can sleep"), it is sometimes criticized, and so that is more common in formal writing ("trying to cut spending so that the deficit will be reduced").

such Some people disapprove of using such as a pronoun ("Such was the result," "sorting out glass and newspapers and such"), but it is now standard.

sure, surely Sure and surely are both standard adverbs, but sure tends to be used informally, whereas surely is more common in formal contexts. When used as intensifiers, sure is highly emphatic and positive ("He sure wants to win this one"); surely generally expresses hopefulness or persuasive intention more than certainty ("Surely they could do it if they really tried").

while The earliest meaning of while, "during the time that" ("She liked to read while he was taking his walk"), is related to time. But while has also been a common synonym for "whereas" ("He believes the story, while I'm not convinced") and "although" ("While he's respected, he isn't actually liked") for centuries, and today these uses are absolutely standard.

whose Whose is often used to mean "of which"—that is, to refer to nonhuman things ("a palace whose furnishings were splendid"). Though some critics believe that "of which" must be used in such situations ("a palace of which the furnishings are splendid"), whose has been used in this way for centuries by good writers and is completely acceptable.

you You is often used in addressing the reader directly ("You should always keep an extra set of keys"), a familiar style that has become very common in recent decades. One can be used instead for the sake of a formal or impersonal tone ("One should always keep an extra set of keys"), but it's used less often than it used to be. The two normally shouldn't be mixed in a single piece of writing. You is also used for addressing a completely indefinite audience—for example, when "You have to consider the long-term effects" is the precise equivalent of the passive "The long-term effects have to be considered."