In Dutch, apostrophes are used to form plurals with s when the noun concerned ends in a, i, o, u or y, as in ski’s, foto’s and hobby’s. By contrast, English just has a straightforward plural with s in these cases: skis, photos, visas etc. An apostrophe here would simply be wrong.
     There is one clear exception, and that is the letters of the alphabet when not capitalized. For example, you must write i’s rather than is in a sentence like there are three i’s in ‘intelligible’. The function of the apostrophe in such cases is to facilitate comprehension.
     There are also a number of cases where both options, with and without the apostrophe, are available. The main categories are abbreviations and decades. Thus one may find CFC’s as well as CFCs, CD’s as well as CDs, 1960’s as well as 1960s.
     Where ease of comprehension is not a problem, the choice between the two forms might be seen as a matter of style, with the apostrophe being the more conservative and the form without the apostrophe being the more modern option. Our advice is not to use an apostrophe if you can help it. In other words, we urge you to follow the strongly emerging general convention that apostrophes (except to mark genitives), full stops and hyphens are predominantly used in the spelling of words to ease comprehension (see Sections 3 and 10) and should otherwise be avoided.