This concept has perhaps caused more trouble than any other aspect of learning English. Parts of speech are myths. They are merely inaccurate representations of the properties of words. The attempt to divide up the dictionary of English into classes of words with similar, predictable behavior is doomed to failure, because it is built on faulty assumptions. There are a very small number of categories of words.
Words belong only to a single part of speech.
In the first three mythbusters we will take each of these and see where the notion of parts of speech fall apart. The grammar of a word is based on its meaning. This is simply not true. There is a strong relationship between the grammar of some words, and their meaning, in an abstract sense, but there are so many exceptions that we cannot rely on such a broad generalization. You may recall being taught that a noun is a person, place or thing and that a verb is an action word.