There are tips and notes for every lesson, but they are only visible in the web app. https://www.duolingo.com/skill/ru/Possessive-Modifiers-1
Tips and notes
POSSESSIVE ENEMY MINE
There isn't much to say about words like "my" or "your" in Russian.
his/her/their do not change: его́, её, их(and they don't get an initial Н after prepositions!)
my/your/our roughly follow an adjectival pattern, i.e. they copy the gender and the case of the noun they describe. Just like этот:
Unlike English, no distinction is made between my and mine, her and hers etc.
Pronunciation: in «его», as well as in adjective endings and "сегодня" the letter Г is pronounced В. It is a historical spelling.
Nouns in Russian belong to one of three genders: feminine, masculine or neuter. If a noun means a person of a certain gender, use that one. For all other nouns look at the end of the word:
(TABLE) ENDING IN NOM; GENDER; EXAMPLES
-а/-я ; feminine ; ма́ма, земля́, Росси́я, маши́на
consonant ; masculine ; сок, ма́льчик, чай, интерне́т, апельси́н
-о/-е ; neuter ; окно́, яйцо́, мо́ре
-ь ; feminine or masculine - consult a dictionary ; ло́шадь, ночь, мать, любо́вь / день, конь, медве́дь, учи́тель
IF THERE'S A SOFT SIGN, IT ISN'T POSSIBLE TO PREDICT THE GENDER, AT LEAST, NOT ACCURATELY. HOWEVER, ABOUT 65-70% OF THE MOST USED NOUNS THAT END IN -Ь ARE FEMININE. ALSO, YOU CAN LEARN THE COMMON SUFFIXES ENDING IN A SOFT SIGN THAT PRODUCE A WORD OF A PREDICTABLE GENDER. THEY ARE:
-ость/-есть, -знь → feminine
-тель, -арь, -ырь → masculine
ALL NOUNS WITH -ЧЬ, ЩЬ, -ШЬ, -ЖЬ AT THE END ARE FEMININE. THE CONVENTION IS TO SPELL FEMININE NOUNS WITH A SOFT SIGN AND MASCULINE ONES WITHOUT ONE: НОЖ, ЛУЧ, МУЖ, ДУШ. IT DOESN'T AFFECT PRONUNCIATION, ANYWAY.
Russian pronouns are inflected for number, case, and gender. This means that masculine, feminine, and neuter words have different forms of "your" (eg, Твой Твоя Твоё in the nominative case) and that each of these changes according to its position in a sentence (ie, the nominative, accusative, genitive, dative, instrumental, prepositional cases). On top of that, there are different forms if multiple people are being addressed (ie, plural forms). This is comparable to French ton/votre or Swedish din/er. Look under "Russian Possessive Pronouns" in the link above to see the whole list of "my", "your", etc.
I wish the lessons were written using cyrillic orthography. They were up until a couple of lessons back, and suddenly they went back to transliteration. This is hard to figure out and at times leaves me confused about the correct spelling. It's funny: the lesson gives the task in the latin alphabet and I respond in cyrillic.
Твой = informal, when speaking to a friend or close one. Ваш = formal, when addressing a stranger (unless it's like a 12 year old) or a group of 2 or more people.
They both decline based on the gender, number and case of the word they are modifying. Here the word is a single neuter word in nominative case, so it requires either твое or ваше. Both of those are acceptable answers, but they just imply that you're talking to two different types of audiences.