It seems that feminine nouns ending in -a lose their endings in the Genitive-Plural case
собака (Nom.) ---- собак
Exactly! The same goes with nouns ending in -я, but in order to keep the last consonant palatalised, the -я gets replaces by the soft sign: тётя --> тёть.
In some instances, after the ending is dropped, a vowel is inserted before the last letter: "many/a lot of cats" = много кошек.
So basically after we say 'много' the noun after it must be in the genitive form?
It's not an ending. «Собак» has zero ending; zero ending shows it's a genitive plural form in this case.
I think you know what plural is, English nouns also can be plural and singular (dogs vs. dog), so only 'genitive' is left to explain.
Russian nouns have several case forms (6 of them, according to the most common classification). Each form is used to convey some grammatical meaning.
Nominative is the most basic form, it's used for the grammatical subject of the sentence (соба́ка сиди́т 'the dog is sitting') and for the predicate in «X is Y» sentences (in «соба́ка зла́я» 'dog is angry', both собака and злая are nominative).
Genitive is often translated with the 'of' proposition, or it's sometimes translated with the English possessive case: соба́ка сестры́ sister's dog (сестры is genitive). However, it has a number of other uses: notably, after «нет» is is used to express absence.
Dative corresponds to the English preposition 'for': соба́ке 'to dog'.
Listing all the cases and their functions here might not be useful, as you'll learn all of them as you progress through the course. To get a list of all the case forms, you could use Wiktionary: https://en.wiktionary.org/wiki/%D1%81%D0%BE%D0%B1%D0%B0%D0%BA%D0%B0#Declension
Oooooh ok thank you, I don't know a lot of plural endings yet so that's why I didn't see it.