1. All were happy.
2. All was clean.
3. All were clean.
Q: In #1 all means all people, so #1 is grammatical. What about #2 and #3?
In #2 and #3, all means all things. In this case, do we have to use 'was' or 'were'?
All of the room was clean. (Singular)
All of the children were clean. (Plural)