First of all, let's have a look at the word classes that are present in modern English. The following classification is adapted and followed by most English dictionaries and consists of eight (sometimes nine) word classes. (Adjective, Adverb, Article, Conjunction, Interjection, Noun, Preposition, Pronoun, Verb)
In many European languages, inflection shows in which category a word belongs. Unfortunately, in English his is a lot more difficult to distinguish. Furthermore, some words can belong to more than one word class, for example Google, which can be used as either a noun or a verb. The following explanations of the different word classes, however, will make it easier for you to distinguish what word belongs to which class.
These word classes can be further divided into two linguistic classes, an open and a closed one. The difference between those is that the open classes allow new word to be invented and added, while there will never be words added to a closed class.
Open classes (Adjective, Adverb, Interjection, Nouns, and Verb).
Closed classes (Pronouns, Prepositions, Conjunctions, and Determiners).