What is the definition of an affix in relation to word modification?

An affix is defined as an addition placed at the beginning or end of a word and serves to modify its meaning or grammatical function. When an affix is added to a root word, it can change the word into a different part of speech or influence its meaning, allowing for greater nuance in communication. For instance, adding a prefix such as "un-" can negate the meaning of a word, while a suffix like "-ing" can indicate the present participle form.

The other options do not accurately capture the essence of what an affix is. Determining a word's part of speech is more related to the grammatical role the word plays in a sentence rather than referring directly to the structural addition itself. A root word without modifications describes the base form of a word, lacking any affixes that alter its meaning. Finally, advanced vocabulary refers to terms that may be complex or sophisticated in nature but does not specifically relate to the concept of affixes. Thus, the definition of an affix is best encapsulated as an addition that modifies a word.