Sometimes I find it confusing when it comes to formal or informal usage of a grammar rule:
Does informal use mean that it is all the time wrong or does it mean it is OK to use?
for example, using "none" with a plural form of a verb or using "both", "all" in negative sentences as in:
None of my friends is here. vs None of my friends are here.
All students don't like maths. vs Not all students like maths.
Can I get some help, sirs?