1. His family is large.

2. His family are all healthy.
3. His family members are all healthy.
4. The members of his family are all healthy.
5. There are 50 families in this apartment building.
6. There are 50 families in these apartment buildings.
7. There live 50 families in this apartment building.
8. There live 50 families in these apartment buildings.
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Are they all grammatical? Does #3 and #4 mean #2? Do you use Sentence 2 in the USA?In British English #2 is okay, isn't it?

I would use these:

1, 3, 4, 5, and 6.

7 and 8 are incorrect unless you rephrase and say, "Fifty families live in .... " (Remember, don't start a new sentence with a numeral; the number word needs to be spelled out.)

Thank you.

2. His family are all healthy.
2-1. His family is all healthy.

[Which one do you use?]

Family is singular. It's one family unit. Use the singular verb form is. Are is for a plural subject as in family members (more than one member).

1.   Which of the following passages from the novel best illustrates the most extreme hostility  between the Chinese and American communities in San Francisco?

Let's analyze each sentence to determine if they are grammatical and if they convey the same meaning:

1. His family is large. (Grammatical and means that the size of his family is big)
2. His family are all healthy. (Grammatical but may sound strange to some native English speakers. It means that each member of his family is healthy.)
3. His family members are all healthy. (Grammatical and conveys the same meaning as sentence 2. It emphasizes that it is the individual family members who are healthy.)
4. The members of his family are all healthy. (Grammatical and means the same as sentence 3. It emphasizes that it is the specific members of his family who are healthy.)
5. There are 50 families in this apartment building. (Grammatical and means that in the mentioned apartment building, there are 50 separate families.)
6. There are 50 families in these apartment buildings. (Grammatical and means that in multiple mentioned apartment buildings, there are 50 separate families.)
7. There live 50 families in this apartment building. (Not grammatical. It should be "There are" instead of "There live." The corrected sentence would be similar to sentence 5.)
8. There live 50 families in these apartment buildings. (Not grammatical. It should be "There are" instead of "There live." The corrected sentence would be similar to sentence 6.)

Regarding the usage of sentence 2 in the USA and British English, it might vary. In the USA, using "family" as a singular noun is more common and may sound more natural. In British English, using "family" as a plural noun (thus using sentence 2) is more acceptable. However, both versions can be understood and communicated effectively in both regions.