Simple. We learnt it at school. How did you think we managed it?

This is because they have learnt the rules of pronunciation at school and from their parents. You can ask this question about any language.

yes. we just know them. I think there are 12 basic vowel sounds in English. For example when I see the letter A, I know that there is more then one meaning depending on the context. This is understandably confusing, English could really use an accurate phonetic spelling system.

practice

Of course not! Not always! But it says Spanish/Italian speakers can most time because to most letters it only has one pronunciation each. Regardless, they have different pronunciations for the site's name "Italki". But yes, the Indo-Eu based languages' speakers can guess how to pronounce a new word because they somewhat more or less have the rules. Edit: I mean and in 99% cases they are right. That's also why most of them don't read the "soundmarks" of their own languages.