Open grammatical categories are usually nouns, verbs, adjectives, and adverbs. There are closed grammatical categories as well, such as pronouns, conjunctions, and determiners. There is no simpler way of creating a new pronoun, conjunction, or determiner since they are part of the syntactic knowledge of a particular language. Hence, when we encounter an OoV form, its grammatical category is likely to be one from the set of open grammatical categories.