This is one of those topics that you can easily understand by understanding the underlying meaning.
For a moment forget about the rules. (just while you read this post).
1A: My money was stolen
1B: My documents were stolen
2A: All my money
was stolen
2B: All my documents
were stolen
3A: Some of my money was stolen
3B: Some of my documents were stolen
I am sure there is no doubt about (1). The author is talking about his money and he is stating that his money was stolen. Same holds true for documents. But notice the difference. Money is singular. - Why? Since it is uncountable noun. You do not say - I have 1 money, 2 moneys, 3 moneys. But documents is plural - why? Since it is countable noun. You say - I have 1 documents, 2 documents, 3 documents.
Now on similar lines, you will be able to get to (2) without any trouble.
Now lets see (3). When the author says "some of the documents", he means more than 1 document - hence it is plural.
However, when the author says "some of the money", even though he means more than 1 money, he cannot say that it is plural since there is nothing like 2 moneys or 3 moneys. This is the underlying reason why (3) is how it is.
Some of the money was & Some of the documents were.
So whenever you have to decide if an item is singular or plural, just see whether you would be able to count that quantity as 1 quantity, 2 quantities, etc. If yes, then it will be plural. If no, then it will be singular.
Now if you want a rule:
For prepositional phrase "some of the noun", the number of verb will depend on the number of the noun. If noun is countable, then verb will be plural. If noun is uncountable, then verb will be uncountable.
Let me throw this in as well:
One of the documents was stolen.
Here the subject itself is only 1 of the many entities. Thus the number of verb will be singular. Again, let the meaning be your guiding light!!
Please let me know if you have any questions.
Regards,
Payal
