1. All of them are useful.

2. All of them is useful.

3. All is expensive.
4. All are expensive.

5. All is silent in the room.
6. All are silent in the room.
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Which ones are grammatical?

All = plural.

Therefore, 1, 4, and 6 are correct.

Out of the six sentences you provided, sentences 1, 4, and 6 are grammatically correct. Here's an explanation for each sentence:

1. "All of them are useful." - This sentence is grammatically correct because the plural pronoun "them" agrees with the plural verb "are."

2. "All of them is useful." - This sentence is not grammatically correct. The subject "all of them" is plural, so it requires a plural verb "are" rather than the singular verb "is."

3. "All is expensive." - This sentence is not grammatically correct. The subject "all" is singular, so it requires a singular verb "is" rather than the plural verb "are."

4. "All are expensive." - This sentence is grammatically correct because the plural subject "all" agrees with the plural verb "are."

5. "All is silent in the room." - This sentence is not grammatically correct. Similar to the previous example, the subject "all" is singular, but the verb "is" should be plural to agree with "all," so the corrected version would be "All are silent in the room."

6. "All are silent in the room." - This sentence is grammatically correct because the plural subject "all" agrees with the plural verb "are."

In summary, sentences 1, 4, and 6 are grammatically correct, while sentences 2, 3, and 5 need corrections to match subject-verb agreement.