As always, I'll keep the explanation as simple as I can, because English doesn't need to be difficult!
You do need to know whether you are using a family's last name in the singular form or the plural form, and whether you're using it as a possessive or not. This is the key.
Let's say my neighbors' last name is Schneider. There are two of them: Mr. Tom Schneider and his wife Mildred. They have a cat and two children.
I live next door to the Schneiders.
I take care of the Schneiders' cat when they're away.
The Schneiders' children are noisy.
Mr. Schneider's sister is visiting this week.
In the first sentence, there is no possessive. The Schneiders don't own anything in that sentence. You need the plural form of their last name, Schneider = Schneiders. There are four Schneiders, and I live next to them all. You don't write "I have four dog's," so don't write that you "live next to four Schneider's."
In the second sentence, the Schneiders DO own something - the cat. The cat belongs to ALL the Schneiders (or they to him, but that's another matter). The apostrophe is needed to show possession.
TIP: When "the" comes before a family name, the name will always be plural, and the apostrophe will always go at the end, following the possessive -s.
In the third sentence, we see the plural possessive again. You see "the" before the name, and the plural form of the name tells you the children belong to both Mr. and Mrs. Schneider.
In the fourth sentence, we have "Mr." instead of "the" before the family name. The sister belongs to only one of the Schneiders, and therefore the apostrophe comes after "Mr. Schneider" and before the possessive -s.
TIP: When "Mr.," "Mrs.," or a first name comes before a family name, the family name will always be singular, and therefore the apostrophe will always be between the family name and the possessive -s.
When nothing comes before the family name, the writer or speaker is just being lazy. What should come before the name - "the" or "Mr./Mrs."?
Do you have Schneiders' phone number? (This is one number for them all.)
Do you have [Tom] Schneider's phone number? (This is one number for one of them.)
I'm going to give Schneiders a call. (No possessive = No apostrophe)
Advanced course:
What if the family name ends with an -s or -ss or -z?!?
Don't panic. Let's take the family name Furness. The concept is the same, except that to make that name plural and refer to ALL members of the Furness family, you need to add -es to the name.
I live next to the Furnesses. (the Schultzes)
I take care of the Furnesses' cat when they're away.
The Furnesses' children are noisy. (the Schultzes')
Mr. Furness's sister is visiting this week. (Mr. Schultz's)
Summing up:
- plural last name alone (no possessive) = no apostrophe
- plural last name + possessive = -s' (apostrophe after the possessive -s)
- singular last name + possessive = -'s (apostrophe before the possessive -s)
WRONG:
Which Berenbaum's what? (found on pintrest, no original link) |
Correct if what is meant is "Marble's cat lives here". WRONG if the cat's name is Marbles.
source |
NOPE:
source |
Correct, IF Jack and Lisa belong to a guy named Weston:
source |
WRONG again, unless this house belongs to "The Drake":
source |
The Furnesses [live here]
The Furness Family [lives here]
The Schneiders [live here]
[This is] The Schneiders' Home
[This is] The Furnesses' Home
I hope that helps!