Different genders and situations...
моя - For Feminine nouns. (Pretty much all words that end in "а" and "я", and sometimes "ь") мой - For Masculine nouns. (Pretty much all word that end in contestants) моё - For Neuter nouns. (Pretty much all words that end with "о" and "е") мои - For Plural nouns.
What is a 'contestant' in Russian grammer? I was gonna be a smartarse and say 'consonants' but then thought better of doing so.
What is the plural form of брат? For some reason we are not taught that in this skill.
It is not taught here because it is irregular, thus it will have to be memorized. Друг and сын make the same plural:
- сын → сыновья́
- друг → друзья́
- брат → бра́тья
- стул → сту́лья
I wanted to beat Theron to it (because I trusted this time I actually could answer a question correctly without making even myself more confused :D).
Is your question about "ё" vs "e"? The correct one is "сёстры", and that's exactly how the word is pronounced. The problem with written Russian though is that "ё" is frequently replaced with "e" in print. (There must be some really silly, antiquated typographic reason for that, similar to the reason why the full stop migrated inside the quotation marks in American English, but I don't know it.) In any case, this is never a problem for native Russian speakers since they know when it's "ё" and when it's "e", but I can imagine that this would drive learners crazy. Sorry!
The reason for that is probably due to typewriters having only E. It's more similar to English loosing Þ, which was due to printing difficulty, than America's punctuation rules, which were just standardised differently.
To add the question, wasn't the singular form spelt with an E? I think that may have been why he was confused.
Actually, the reason for American placement of commas or full stops before closing quotation marks (regardless of the logical structure of a sentence) is also rooted in typesetting (which is what I was referring to): http://grammartips.homestead.com/inside.html - check the footnote about the origins of this rule.
Now, to answer your question, the singular form, сестра, is indeed spelt (and pronouced) with an E, not Ё. Making it plural changes both the spelling and the sound. I don't think this is common in Russian, but you find it all over the place in German, and English man → men, woman → women may also be examples of that. Given that "сестра" is a very old word (and for that reason it is so similar in many Indo-European languages), I suspect this may be a remnant of some ancient way of making plurals.
Ё in Russian arised as a replacement of the stressed /e/ vowel every time it was after a palatalized sound but not before another palatalized sound.
So every stressed е that comes from old "e" went through that change under these circumstances. Those е's than were Ѣ didn't. No wonder that when a word has a shifting stress, some words have their "е" replaced by Ё.
What is so irregular about it? Вода́→во́ды, река́ → ре́ки, рука́ → ру́ки, жена́ → жёны. :)
Well... I guess half Russian nouns are irregular, lol. In most cases you should memorize the correct form.
Isn't Ж always hard? So why жена́ → жёны? That's not an е after a palatalized sound. Oh, or did you mean that жёны was originally stressed on the last syllable, but later started being stressed on the first, so the е was changed to ё just to reflect that?
You must understand that all hushes were once soft (having been born from palatalized к, г, and х). Spelling conventions still reflect that to a degree. Though, you may just as well say that the spellings are arbitrary (but consistent). Indeed, if a consonant is always hard or always soft, it does not matter (in term of pronunciation) which vowel you select to spell the vowel sound after it.
An unstressed о is not spelt after a hush. By convention, you always spell е instead.
As for the unexpected Ё, this sound started replacing a stressed Е more than half a millenium ago in certain positions (after a "soft" consonant while before a "hard" consonant). It happened before Ж and Ш turned perma-hard, so you have a number of words like чёрный, шёпот, жёлтый, щётка, расчёска, расчёсывать, чёрт. Of course, moving stress also can make ё appear or turn back to orthographic е.
Do they have to be literal sisters, or can it be used to refer to female friends?
Yes, vast majority of time. There could be exceptions, like with most things in life (say, group of girls can sometimes refer to themselves as "сёстры / сестрички / сестрёнки"), but these are rare.
In religious context one can say "sister" about any girl/woman of the same religious community. And in Orthodox tradition sometimes it is pronounced as "сестры" instead of "сёстры".
should this not be сестри? I thought nouns ending in -a would be feminine. I guess this is just an irregular case?
It is feminine (no surprises there, it's sisters we are talking about!). However, in every single example of a feminine noun ending with -a I can think of, with the exception of words ending with -га, -ка, -ха, -ча, -жа, -ша, ща, -a becomes -ы when you make the word plural. (In words with those "exceptional" endings, it indeed becomes -и.)
Generally, "hard becomes hard" (а -> ы) and "soft becomes soft" (я or ь -> и), unless there is a spelling rule that prevents a hard ending from staying hard (for instance, "госпожа" becomes "госпожи" because you can't have an ы after a ж).
Сестри is therefore not a valid plural spelling for сестра. The spelling of words is very exact and there typically isn't an option in how something is spelled.
The e has an umlaut over it in the Duo sentence. That changes the pronunciation.
Моя = feminine Мои = plural (for feminine, masculine and neuter nouns) Мой = masculine Моё = neuter