More Than One
Most nouns are made plural simply by adding s (or es if the word already ends in s or sh). So more than one dog are dogs, one horse plus another horse makes two horses, and you can be in one skirmish or several skirmishes. Then there are the irregular plurals like mouse becoming mice and knife knives, but that doesn't concern us here. The point to remember is that when you add s to make a plural, you add only the s, never apostrophe-s. So dogs is always dogs and never dog's if you're talking about more than one dog.
But That Is Mine
Nouns are made possessive by adding s also. This is why people get confused. But it doesn't have to be confusing if you remember that to make a noun possessive you add apostrophe-s, and not just s all by itself. So to say that a bone belongs to a dog, you say that it is the dog's bone. A horse that belongs to Jonathan is Jonathan's horse. And so on.
But then what do you do if the noun or name ends with s already? There are two possibilities. One is to go ahead and add apostrophe-s after the s that is already there. The other is to just add an apostrophe. So you could say that a car belonging to Seumas is Seumas's car, or that it is Seumas' car. Adding both the apostrophe and the s is generally considered more correct, though either option is acceptable. The same two possibilities are available when making a plural noun possessive. You could say the dogs' bone or the dogs's bone in order to indicate a bone belonging to several dogs. To talk about the house where the Joneses live (more than one person with the last name Jones), you would say the Joneses's house or the Joneses' house. Writers usually use whichever is most like the way someone reading aloud would pronounce it; so a writer would probably write Seumas's car but the dogs' bone and the Joneses' house.
More Apostrophe-ses
Another place you get that old apostrophe-s is when you contract two words into one word (known as a contraction). Contractions don't always involve ses, of course; don't and isn't are contractions. But it's the ses that get confusing. All you need to remember is: in a contraction, the s always stands for another word (usually is, but sometimes other words like us). So that dog is running can become that dog's running and let us go swimming becomes let's go swimming
Most nouns are made plural simply by adding s (or es if the word already ends in s or sh). So more than one dog are dogs, one horse plus another horse makes two horses, and you can be in one skirmish or several skirmishes. Then there are the irregular plurals like mouse becoming mice and knife knives, but that doesn't concern us here. The point to remember is that when you add s to make a plural, you add only the s, never apostrophe-s. So dogs is always dogs and never dog's if you're talking about more than one dog.
But That Is Mine
Nouns are made possessive by adding s also. This is why people get confused. But it doesn't have to be confusing if you remember that to make a noun possessive you add apostrophe-s, and not just s all by itself. So to say that a bone belongs to a dog, you say that it is the dog's bone. A horse that belongs to Jonathan is Jonathan's horse. And so on.
But then what do you do if the noun or name ends with s already? There are two possibilities. One is to go ahead and add apostrophe-s after the s that is already there. The other is to just add an apostrophe. So you could say that a car belonging to Seumas is Seumas's car, or that it is Seumas' car. Adding both the apostrophe and the s is generally considered more correct, though either option is acceptable. The same two possibilities are available when making a plural noun possessive. You could say the dogs' bone or the dogs's bone in order to indicate a bone belonging to several dogs. To talk about the house where the Joneses live (more than one person with the last name Jones), you would say the Joneses's house or the Joneses' house. Writers usually use whichever is most like the way someone reading aloud would pronounce it; so a writer would probably write Seumas's car but the dogs' bone and the Joneses' house.
More Apostrophe-ses
Another place you get that old apostrophe-s is when you contract two words into one word (known as a contraction). Contractions don't always involve ses, of course; don't and isn't are contractions. But it's the ses that get confusing. All you need to remember is: in a contraction, the s always stands for another word (usually is, but sometimes other words like us). So that dog is running can become that dog's running and let us go swimming becomes let's go swimming

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