Final answer:
The word 'crammed' in the given sentence is a verb, depicting the action of passengers filling up the subway car. It is important to identify the functions of basic parts of speech like nouns, adjectives, verbs, and adverbs in sentences.
Step-by-step explanation:
In the sentence 'More passengers crammed into the crowded subway car.', the word crammed serves as a verb. In this context, crammed is the action that the passengers are performing - they are actively filling the subway car to capacity. Parts of speech are essential components in sentences, and in English, the four basic ones are nouns, adjectives, verbs, and adverbs. Understanding how they function within a sentence is crucial to mastering the language.