TO INDICATE POSSESSION:
1. Possession may be expressed equally well by "of" or by using the apostrophe and shifting the word order.
For example:
The dress of the girl; the girl's dress.
So too for a plural possessor: The dresses of the girls; the girls' dresses.
2. The rule is to put the apostrophe after teh possessor and add an "s" if the possessor is in the singular:
this girl's dress
the class's teacher
this horse's hooves
3. If the possessor is in the plural, simply put the apostrophe after the plural possessor:
all the girls' dresses
all the classes' teachers
all the horses' hooves.
This rule for a plural possessor remains the same with both a singular and a plural object of possession. So the idea of all the students and the ideas of all the students will both be expressed with an apostrophe as all the students' idea and all the students' ideas.
4. The only exception to these rules for the use of the apostrophe indicating possession is for personal pronouns, which do not have an apostrophe:
his, hers, theirs, ours, yours, its
NOTE: It's is a contraction from it is or it has and is NOT the possessive.
5. Proper nouns (names of people, places etc and usually capitalised) that end in "s" are sometimes given another "s" as if they were singular _ as in (2) above _ and sometimes not as if they were a plural. You may say either:
Keats' poetry or Keats's poetry
Jesus' life or Jesus's life
6. The apostrophe should NOT be used for plurals without possession.
TO INDICATE ABBREVIATIONS:
1. The apostrophe is used in writing and printing to indicate that two words are abbreviated or contracted from their fully written out and correct spelling. Usually this echoes the greater rapidity or casualness of spoken language in connection with verbs.
(i) The most common use is with pronouns and verbs:
I'm (I am), you're (you are), we're (we are), they're (they are), she's (she is), he's (he is), it's (it is or has), I'd (I had or I would)(ii) Contractions of negative verbs may produce other alterations in the shortened form:
don't (do not), can't (cannot), won't (will not), didn't (did not), shan't (shall not)(iii) Contractions of sequences of verbs are common:
could've (could have), would've (would have), should've (should have)WARNING: Some of these shortened verbal clusters sound rather like other word groupings. It is a major offence to write could of instead of could've, would of instead of would've should of instead of should've.
(iv) You should not use abbreviations in formal writing, including academic writing and most journalism.
2. Sometimes writers try to copy the natural dropping of sounds in informal spech, especially in dialects, and indicate this elision by using an apostrophe for the missing letters: "Th' ol' ca' wi' two ki'ens is cryin' t'nigh'."