What is a key function of a compound word?

A key function of a compound word is to combine the meanings of two root words. When two words are joined together to create a compound word, they form a new word that encapsulates the meanings of both original words. For example, in the compound word "toothbrush," "tooth" and "brush" come together to refer specifically to a brush used for cleaning teeth. This combination allows for more nuanced and descriptive language, enhancing communication by creating specific terms that represent distinct concepts.

The other options imply functions that aren't characteristic of compound words. Changing the tense of a root word, altering speech, or serving as an inflectional marker pertains more to grammatical modifications rather than the creation of new meanings through the combination of words, which is the essence of what compound words achieve.