Hi, Scarlett!
First, I want to point out they are not GRAMMAR questions at all. The questions you are asking are pronunciation and spelling questions.
Grammar is a set of rules how to combine words into a sentence, how to form a question, a negation etc., how to form plural, cases, genders, verb tenses, aspects etc., how and when to use them. I hope you get the idea
Then,
1. Yes, it has to do with the vowel reduction as iCake pointed. What you have to know is the right stress position in each word. Please note that in most of cases there are no any clear signs in a word spelling to indicate which syllable is stressed. You just have to know it "by ear".
But I can provide you with some examples from English to clear it up. BTW, is your native language English?
In words like "seldom", "melody", "music", "traffic", "pilot", "sentence" etc. it is the FIRST syllable which is stressed.
In words like "ago", "ahead", "begin", "submit", "conclusion", "provide", "defend" etc. it is the SECOND syllable which is stressed. Try pronouncing them with the stress shifted to the first syllable and you will feel it is wrong.
The only difference is that in most of English words the stressed syllable is either FIRST or SECOND. But in Russian, any of the syllables can be stressed. Some words have their stress on the first syllable (I mark them red: книга, стадо, облако, радуга, истинному, высказано etc.); some words have their stress on the second syllable (окно, стена, дорога, собака, заманчиво, которому, указанному, высказывала); some words have their stress on the third syllable (колесо, берега, передача, показала, настоящая, упаковано etc.); some - on the fourth syllable (катамаран, переходил, Бородино, сопротивление etc.); some - on the fifth syllable (распереживался, достопримечательность) - this case is already rare, etc.
The vowel which is stressed is always clearly announced, it is usually longer than unstressed vowels, stronger, and clear. All unstressed vowels are short, weak and usually unclear (that is what the reduction means). In English, "current" and "currant" sound the same due to the vowel reduction in the second (unstressed) syllable. The first vowel in "begin" sounds as in "big", not as in "beg", doesn't it? And that is because of the vowel reduction in the first (unstressed) syllable.
The same we do in Russian. Only the rules of reduction are somewhat different. "А" and "О" are clearly distinguished only when they are stressed (маг - мог), but when unstressed, both "А" and "О" are reduced to a weak "а" sound (something similar to the English "uh"). Therefore, "cама" and "сома" are pronounced the same. The first vowel is a weak unclear "а".