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.
I don't know, it just doesn't work. There is some notion of what sounds “good” and what doesn't, and if I try to form oblique forms, they don't sound good at all. It's probably related to why you can't say “I will win” with «победить» or “34 watches” with «часы», but “we will win” and “35 watches” are absolutely fine.
On the other hand, there is some tendency to decline the undeclinable (because undeclined forms sometimes sound awkwardly imprecise), so it's not impossible to hear «нет там жалюзей»... which is absolutely unacceptable as far as dictionaries are concerned (and is really bad Russian, so don't do that), but phonetically is OK.
But I'm an armchair linguist, so take this with a cubic metre of salt.
The same in Polish, some time ago words "radio", "metro", "kakao" were undeclinable. Now it is changing, "radio" and "metro" decline even in dictionaries - but "kakao" is still undeclinable (some people say "kakaa", "kakaem" - but it can be acceptable "only in very informal Polish". I think these words were treaten as foreign in Slavic languages, but now they have been used for such a long time that for people they became "native" and that's why they decline them (prof. Jan Miodek, a popular Polish linguist, writes it is a natural process). In Polish it already happened, maybe in Russian it will take more time? But if now these are bad forms, we should learn as good Russian as we can ;)
You could use твои (rhymes with "i-ee", used for plural nouns) (not твой, which rhymes with boy and is for masculine nouns) if you were talking to an individual with whom you were already familiar, like a friend. Ваши works if you're talking to two people (like both parents) or are being formal.
Use твой when you'd use ты (speaking to one person with whom you are familiar); use ваш when you'd use вы (speaking to multiple people, or when addressing someone formally like a teacher, supervisor, stranger, etc.).
Твое is for singular neuter nouns in the nominative case; ваше is for singular neuter nouns in the nominative case.
Because in English you either have to say "a taxi" or "the taxi", with an article, if you're talking about a single taxi. In plural, you can say "They are in taxis", though it sounds a bit odd just because of the meaning of the sentence. Russian does not use articles in this manner.