Learn Russian: Это брат. - This is a brother.

In Russian, you generally don't use a verb for "is" or "are" in the present tense. You simply put the subject and the noun together, so Это брат literally translates to "This [is a] brother."

Why isn't there a word for "a" before брат?

Russian doesn't have articles like "a", "an", or "the". The word брат can mean "brother", "a brother", or "the brother" depending entirely on the context.

Since брат is a masculine word, shouldn't это change to a masculine form?

Not when you are using it to introduce or point something out by saying "This is..." or "It is...". In this specific "pointing-out" structure, это acts as a fixed starting word and always stays in this default form, no matter the gender of the noun that follows.

Why does the "o" at the end of это sound like an "a" when spoken?

In Russian, when the letter о does not have the stress (the spoken emphasis), it is pronounced like an "a". In это, the stress is on the first letter (Э), so the final о sounds like a short "a".