No. I believe this holds for feminine words ending with -a/-я when those are preceded by a consonant. On the other hand, -ия/-ея becomes -ий/-ей:
одна Мария - пять Марий, одна лотерея - пять лотерей. And even when -a/-я is preceded by a consonant, but there is more than one, an extra vowel may "sneak in" before the last consonant to make the pronunciation easier: одна палатка (one tent) - пять палатoк.
I guess you can say that it's always the case that the vowel disappears, and the ending is removed. However, when the ending was soft you still want to retain that softness.
So with Мария, you would get Мари but you want to keep that extra 'j sound' at the end and change it into Марий.
The same goes for e.g. неделя - недел - недель, in this case the pronunciation between недел and недель is quite different and because the original word is soft you want to go for недель. You can interpret the 'ь' as being the 'no ending for soft consonants'.
That interpretation might be an easy way to remember the endings?
I dont understand why Genetive Plural lessons dont open by defaul when we need that skill to follow..
The genitive plural was mentioned in the notes of Animals I skill:
Later you will encounter the Genitive plural (often used with numbers and words like "many" or "few"), which shows a simple pattern for -к-suffixed feminine nouns that do not have a vowel before "-ка":
много кошек = many cats
много девочек = many girls
много уток = many ducks
много тарелок = many plates
As you can see, the vowel (О or Е) depends on whether the previous consonant is palatalized or not. Hushes behave as if they were palatalzed, despite Ж and Ш having lost this quality in the modern language.
The default rule for a feminine word which ends in -a (кошка) in genitive plural is to drop the a. For example, собака becomes собак. This would mean that кошка becomes кошк.
However, then there's another rule saying that if the feminine word now ends in a double consonant (as it does here), quite often a vowel (o or e) is inserted between the consonants for easier pronunciation. In this case it becomes кошек.
Another example is девушка - девушек.
Not quite. When using numbers not ending with words "один", "два", "три", "четыре", you use plural genitive. With numerals ending with "один", use singular nominative; with numerals ending with "два", "три", "четыре", use singular genitive:
10 сats - десять кошек
21 cats - двдцать одна кошка
34 cats - тридцать четыре кошки
14 cats - четырнадцать кошек (contrary to the appearance, the number does not end with the word "четыре"!).