Structure-class words are mostly closed, relatively small groups of words. Words like of, out, his, and the cover all the grammatical bases and can be used to describe any relationship between form-class words. There’s no need for new structure-class words. For this reason, as a class, structure-class words have mostly grammatical meaning. For example, the word the is a key marker by telling us a particular noun will follow in the sentence, and they generally do not change form, taking neither inflection nor derivation. This also means that the form and function of a structure-class word is always the same. The form and function of a determiner is always determiner; the form and function of an auxiliary is always auxiliary; and so on.

There are nine sets of STRUCTURE-CLASS words described in this sub-section:

1. DETERMINERS
2. PRONOUNS
3. AUXILIARIES
4. QUALIFIERS
5. PREPOSITIONS
6. CONJUNCTIONS
7. SUBORDINATORS
8. RELATIVES
9. INTERROGATIVES

NOTE: there are two words that we do not cover in this section, that don't really fit into any category of structure-class word, but we oftentimes need to account for them:

• NOT: for our purposes, we are not going into detail on the use of the negative in the English language. If you are asked to identify NOT (or similar word) as part of an exercise, you can simply refer to it as Negative.
• TO (the infinitive marker): if TO appears before a verb in its base form, it is the KEY MARKER for an infinitive phrase. If you are asked to identify this TO as part of an exercise, you can simply refer to it as Infinitive Marker.