What is the difference between S' and 'S?
When can we use S' and when can we use 'S??
Both express possession, of course.
We use 's with singular nouns. For example, "my son's toys" will be "the toys that belong to my son".
We use only an apostrophe (') after plural nouns that end in -s: "my sons' toys" means that I have more than one son and these are their toys.
We use 's for possession with the other plural nouns. For example: "my children's toys; women's wishes, etc.
Fluffy's answer is correct about the possessive usages, so I won't repeat those. However, it misses an important point.
Possession isn't the only use for 's; it can also be a contraction for is. For example: it's, how's, he's, she's, that's, etc. In standard English, s' is never a contraction*.
The double meaning of 's leads to one of the most common mistakes in written English: confusing its, which is the possessive form of it, and it's, which is a contraction of it is.
* - When approximating certain dialects or slang in writing, s' may be used, rarely, as an abbreviation or contraction of it is when placed at the front (never the back!) of another word. For example, there's a famous comedy puppetry bit which makes heavy use of s'alright to mean it's alright. However, this is absolutely neither standard nor something you should emulate.
First and foremost -- considering that this site is for people who are learning English and who are mostly at the beginning of that process -- you need to understand this: s' almost NEVER appears... and even people whose mother tongue is English will accidentally use 's when they should use s'... it's a VERY common error. So forget about it and focus on other issues.
Don't get me wrong: I don't mean to suggest that this distinction doesn't matter, but rather just this: to get by... you can always use 's instead of s' and you will be making a mistake about 1% of the time and when you do, very few people -- possibly nobody! -- will ever notice.
Take away: be confident and use 's rather than s' all the time. Expect to master this only after you have mastered almost everything else.
"s'" is for multiple, shared, common object, likewise, "parents' night", in other side, it could mean, "the night of parents".
"'s" is for single specified object, for instance, "Ben's dream", could mean, "the dream of Ben", or, it could be a abbreviation of "is", in American English, likewise, "Jack's dead!", which is actually meaning that Jack is dead.
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