Is there a rule or something important I need to remember that may help me to find out if a noun is countable or uncountable?
A countable noun can be written with a number and is usually in plural then (three men, six dogs etc,)
An uncountable noun can only be in singular and cannot be counted. (e.g. water, bread, hate)
A countable noun is something we can count so we can write it with numbers and in plural (three men, six dogs)
An uncountable noun is a noun which does not have plural form and cannot be written with numbers without auxiliary words. I mean that you cannot write for example six waters, but if you add some "units" then the phrase would be correct (for example three bottles/litters/glasses of water).