The concept of parts of speech (classes of words such as noun, verb, adjective, etc.) is confusing because there are many ways to describe parts of speech. One way is to say what they mean, e.g. a verb is an action. Another way is to say how they are used, e.g. a verb is the main part of the predicate. A third way is to say what various forms the words have. e.g. a verb is a word to which you can add -s, -ed, and -ing, e.g. work, work-s, work-ed, work-ing. This third way is how parts of speech are defined in this Grammar.
In English the concept of parts of speech is particularly confusing because English is very complex in this regard; a single word may function as all three major parts of speech. The word market functions as a noun in the sentence 'The market is open.' It functions as a verb in the sentence 'They market their goods in Boston.' And it functions as an adjective in the phrase 'market economy'. Russian does not have this complexity.
In Russian it is most often the case that the form of a word gives you some hint as to what part of speech it belongs to. You can therefore forget the problems you may have had with terms like noun, adjective, and verb in English grammar and start to think of these terms in a new and simpler way.