- What's the difference between
- What rules should I follow to know which one to use?
- Could you provide examples showing their uses?
Más means 'more'
Mas means 'but', usually it can be replaced by 'pero'
They both have the same pronunciation(*) despite the accent, the accent is here to make clear what meaning is actually used. Note that 'mas' is uncommon in a casual conversation, it is mostly used in literature.
(*) That's only true if they are alone. In a complete sentence, the pronunciation may vary. Please, see Javi's comment for more details.
When you use "más" it means more.
When you use "mas" you are contrasting ideas, like "but".
"Mas" is rather unfrequent - at least in every day speech, here in Argentina, but I suspect it's the same in many other places, based on movies, TV, books, etc. It even sounds a bit archaic.
You almost always will see and want to use "más", not "mas" (most of the times you read "mas", it's probably a "más" missing an accent).
How to tell them apart? A quick rule (that I'm making up right now so it's obviously not thorughly tested!) is remembering that "mas" means "but". So if you can replace it with "pero", it's "mas"; otherwise, it's "más".
A dumb example of both words in a sentence: