To answer your second question, ALL nouns starting with a vowel sound use "an" and ALL nouns starting with a consonant sound use "a". There are no exceptions, but you need to remember that it's the pronunciation that's important, not the spelling. If you think you've found an exception, let me know and I'l explain it.

Do an online search using these keywords: "use in on at grammar" This question has been answered and explained thousands of times. :) However, if you have a specific question with a specific example, you can ask about it in another question.

a e an, tem algumas exceções que me confundem também, como "a university", " an hour", "a hotel", etc. Mas no geral, a é usado antes de substantivos que iniciam com som consonantais e an é usado antes de substantivos que iniciam com som vocálicos.

Também tenho essa dificuldade. Também me confundo com a e an