"The girls are eating apples."
Translation:Девочки едят яблоки.
That's conjugation. It exists in quite a few European languages, but not english for the most part. In other languages, the ending of a verb changes depending on who's doing it. The only example of this in english is how when "he/she" is doing the action the end of the verb changes. I'm not very proficient in Russian, but as an example here are the different verb endings in French for manger (to eat)
Je mange (I eat) Tu manges (You eat) Il/elle mange (He/she eats) Nous mangeons (We eat) Vous mangez (Y'all eat) Ils/elles mangent (They eat)
For Russian, what we know right now is
Я ем (I eat) Он ест (He eats) Они едят (They eat)
"Яблоки" is the proper plural of the word "яблоко", I am guessing it's an exception to the rule you cite - I can't think of another one right away. The word closest in sound that comes to mind is "облако" - a cloud, the plural of which is "облака" (with the stress migrating from the first to the last vowel).
I have an idea. In Russian, most of fruits or berries and their trees or bushes has the same name, i.e. "груша" is both a pear and a pear-tree. The same is for "слива", "абрикос", "персик", "рябина", "калина", etc. However, the fruit of an apple-tree and the apple-tree itself have different names: "яблоня" and "яблоко". The plural form of "яблоня" is "яблони". Maybe the plural form of "яблоко" was influenced by the plural form of its tree.
For smartphones, Go to your keyboard's language settings and download the Russian keyboard. I use Gboard (Google keyboard) so, for me I held the globe button for a second to show the languages installed. And below the languages was the button for the language settings. Tap. A new screen pops up, tap on add keyboard at the bottom. Find your keyboard. To change your keyboard, hold or press the globe button again.
This is in fact not an exception. The rule states that all neuter nouns that end in -ко get the plural ending -ки. Облако -ка is an exception to this rule (among others). I admit that it sounds confusing that an exception to a rule gets the regular ending to the majority of the neuter nouns.