We use the present simple to talk about things and situations that are generally true

Form Classes and Structure Classes

"[The] distinction between lexical and grammatical meaning determines the first division in our classification: form-class words and structure-class words. In general, the form classes provide the primary lexical content; the structure classes explain the grammatical or structural relationship. Think of the form-class words as the bricks of the language and the structure words as the mortar that holds them together."
The form classes also known as content words or open classes include:
Nouns
Verbs
Adjectives
Adverbs
The structure classes, also known as function words or closed classes, include:
Determiners
Pronouns
Auxiliaries
Conjunction
Qualifiers
Interrogatives
Prepositions
Expletives
Particles
"Probably the most striking difference between the form classes and the structure classes is characterized by their numbers. Of the half million or more words in our language, the structure words—with some notable exceptions—can be counted in the hundreds. The form classes, however, are large, open classes; new nouns and verbs and adjectives and adverbs regularly enter the language as new technology and new ideas require them."(Martha Kolln and Robert Funk, Understanding English Grammar. Allyn and Bacon, 1998)

One Word, Multiple Classes

"Items may belong to more than one class. In most instances, we can only assign a word to a word class when we encounter it in context. Looks is a verb in 'It looks good,' but a noun in 'She has good looks'; that is a conjunction in 'I know that they are abroad,' but a pronoun in 'I know that' and a determiner in 'I know that man'; one is a generic pronoun in 'One must be careful not to offend them,' but a numeral in 'Give me one good reason.'"(Sidney Greenbaum, Oxford English Grammar. Oxford University Press, 1996)