The treatment of roots, prefixes, and suffixes pertains to the simple union of syllables into complete words, and belongs to morphology.
Words in the five classes that can change their form are called declinable. Those that have only a single form are called indeclinable.

For example

The words friend, friend-s; love, lov-ed are declinable; that is, they are words that can take on several different forms such as with or without an ‘s’ ending.
The words and, but, to, and upon are members of the indeclinable parts of speech.

Parts of Speech Defined

In English, there are eight classes of words called the parts of speech. These are:

• Nouns
• Adjectives
• Pronouns
• Verbs
• Adverbs
• Prepositions
• Conjunctions
• Interjections

Nouns, adjectives, pronouns, verbs, and adverbs are declinable. Prepositions, conjunmctions, and interjections are indeclinable.