Four parts of speech can be classified with a binary system which uses the symbols N and V. They stand for noun and verb. These four parts of speech are nouns, verbs, adjectives and prepositions.
Pronouns are similar to nouns in the sense that they replace them and can be classified with nouns as substantives. Adverbs are closely associated with verbs because they modify them. Conjunctions and interjections are not as great in number as other parts of speech but have very useful functions. Conjunctions serve to combine clauses and interjections to express a wide range of emotions.
Since every sentence must consist of a noun and a verb, it is logical to use the symbols N and V to represent nouns, verbs, adjectives and prepositions. In the imperative "Sleep!", no noun is present, but the pronoun "you", a substantive, is understood.
Nouns can be classified as +N, -V. Therefore, verbs can be classified as -N, +V.
In this system, the classification of nouns and verbs is clear. Nouns and verbs share opposite features. However, the classification of adjectives and prepositions is less clear.
Adjectives can be similar to nouns. For example, the sentences "I'm a Canadian" and "I'm Canadian" are similar. In the first sentence, the complement "Canadian" is a noun and in the second it is an adjective. This provides evidence that adjectives are similar to nouns.
Adjectives can also be similar to verbs. In Japanese, the adjective for cold is "samui". However, it was cold is "samukatta" in which a past tense verb ending is suffixed to the adjective. This same verb ending can be seen in the sentence "Wakatta" which means I understood. The pronoun does not need to be expressed but if emphasis is needed, one can say "Watashi wa wakatta". The particle "wa" is a subject marker. This example illustrates that adjectives can also behave like verbs. For this reason, adjectives are considered to have properties of both nouns and verbs.
Since adjectives can behave as both nouns and verbs, they are classified as +N, +V. This leaves prepositions, parts of speech which express the relationship between two nouns. For example, the sentence "The car is on the road" expresses the relationship between the nouns "car" and "road". Prepositions are functional parts of speech unlike nouns, verbs and adjectives which have lexical meaning. They are so different from nouns and verbs that they are classified as -N, -V.
The binary system with the symbols N and V can be used to classify four parts of speech. This system highlights the importance of nouns and verbs, the properties which adjectives share with them, and the very different nature of prepositions from nouns, verbs and adjectives.