- 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'
No quiero más comida (I don't want more food)
Mas means 'but', usually it can be replaced by 'pero'
Fui a casa, mas estaba vacía (I went home, but it was empty)
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. As written in some comments:
They are pronounced in the same way when they are alone.
Mas is "átona" and más is "tónica", so they're not pronounced exactly the same.
Words that are "tónicas" are stressed in the context of a sentence while the "átonas" are linked to whatever "tónica" word appears next in the sentence (forming a group that is treated like a "whole word" in pronunciation). Here is a good article about that: blog.lengua-e.com/2011/palabras-tonicas-y-palabras-atonas
"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:
Las dos son palabras perfectamente válidas, mas una se usa mucho más que la otra.