通常来说，我们不用撇号来表示复数形式，犯这种错误会被叫做greengrocer's apostrophe, since grocers are often the worst (or at least the most visible) offenders.如果你有很多苹果，用apples而不是apple's.   和缩写词和年份一起时的使用方法。

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The wrong use of an apostrophe to form the plural is called the greengrocer's apostrophe, since grocers are often the worst (or at least the most visible) offenders.  If you have more than one apple, then write apples, not apple's.  An occasional exception to this use is in the case of making a single letter plural.  Therefore, Why are there so many i's in the word "indivisibility"? is correct, depending on who you ask.  This is simply for clarity reasons, so the reader does not mistake it for the word "is."  However, in modern usage, the preference is to avoid inserting an apostrophe and instead surround the single letter in quotation marks before pluralizing it: Why are there so many "i"s in the word "indivisibility"?  Avoid the problem altogether with small numbers by spelling out the word: "ones" instead of "1's," "fours" instead of "4's" or "nines" instead of "9's." Only spell out words of numerals that are ten or lower. Say you use an acronym for a noun, like CD.  To make CD plural, use "CDs," not CD's."  The same logic goes for years — instead of writing "Spandex was popular in the 1980's," use "1980s." The only time an apostrophe should be used in a year is if it's standing in for omitted numbers.  For instance, if you wanted to shorten the year 2005, you could write '05.  In this case, the apostrophe is essentially acting like it does in a contraction and serving as shorthand.