Hello, dear student!
Today we are going to deal with a difficult task which actually always causes difficulties with understanding of those who learn the Russian language. It is the changing of a word stem. It raises many questions and it's rather difficult to tackle this issue by your own. But let's try to analyze and solve this problem.
Some suffixes are added, disappeared or interchanged in nouns. Often plural nouns have this phenomenon. For example, masculine and neuter nouns in plural can make 2 stem-forms at once:
Колы - колья [kaly - kol'ya] - stakes, зубы- зубья [zuby - zub'ya] - teeth - blades.
To understand what form to use in our speech, we should know its meaning. For instance, we use the word "зубы" (teeth), when we want to say about bony body in our mouth and which we use to chew. And the word "зубья" (teeth/blades) is used in Russian, when we are talking about something like a saw. A saw has its own blades - sharp formation that we use to saw something. We call such words - "паронимы" [paronimy] - paronyms. There are special dictionaries where you can find meaning of any words, if you are not sure in their usage or meaning. Sometimes words differ stylistically i.e. we use one or another word in Russian depending on their context. For example, if you see/hear the word "крылья" [kryl'ya] (wings) in colloquial language - "взмахнул крыльями" (flapped the wings), - you can see the word "взмахнул крылами" in more formal or literary speech.
You can often see such words as "горожанин" [garazhanin] - a citizen, "крестьянин" [krist'yanin] - a peasant, "армянин" [arm'yanin] - Armenian in the Russian language.
To decline these words correctly, you should remember that in the process of declension they change their form in plural, for example:
They lose the part -ин [in] and have the ending -e [e] in nominative case and zero ending in genitive case:
Горожанин - горожане (plural, nominative case) [garazhanin - garazhane] - A citizen - citizens