Which word class is often defined by position rather than form?

The word class that is often defined by position rather than form is prepositions.

Here's why:

* Form: Prepositions don't have a specific form or ending that distinguishes them. They can be single words (e.g., "on", "in", "to"), or they can be phrasal prepositions (e.g., "according to", "in front of").

* Position: Prepositions are typically found before a noun phrase or pronoun, indicating the relationship between the noun phrase and another element in the sentence. This position is what helps to identify them.

For example, consider the following sentences:

* The book is on the table. ("on" is a preposition indicating location)

* I went to the store. ("to" is a preposition indicating direction)

In both sentences, the prepositions are placed before the noun phrases ("the table" and "the store") and reveal their function in the sentence.