Using an apostrophe to show omission is quite easy. If you leave letters out of words, then show you have left them out by putting an apostrophe instead. However, in academic English, try to avoid the use of contracted (shortened) words. Use the full forms where possible.
- He is → he's
- is not → isn't
- will not → won't
- you are → you're
- she will → she'll
- etc. etc.
One particular problem is the omission of 'i' from "it is":
- it's = it is (or sometimes it has)
In other words, "it's" never indicates possession: this is an exception to the use of the apostrophe to indicate possession.
Note: plural nouns which are NOT possessors of anything (simple plurals) never need apostrophes. This applies to
- abbreviations: the family doctor is a G.P.; her colleagues are G.P.s
- dates: 1980 + 1981 + 1982 ... + 1989 are the 1980s, with no apostrophe - although that incorrect use is to be seen in some academic writing.
- other numbers. (A bingo caller announces "All the 2s" for 22.
- You may also want to see apostrophe (possession), about the use of the punctuation mark to indicate the relationship of 'belonging to'.
- For apostrophe as a technical term in rhetoric and similar studies, see apostrophe (rhetoric). (Pedants will insist that the punctuation mark the apostrophe should be pronounced distinctly from the rhetorical apostrophe, as it was adopted in English from the French, where it has three syllables, 'a-po-stroff' (IPA: /a pɒ strɒf/. But the French word too is derived from Greek στροφὴ ('strophe') 'a turning', through άπὀστροφος, an adjective meaning 'of turning away', or 'of elision', so all but pedants may ignore this.)