In the sentence "If your bicycle tire is low, use a pump to fill it with air.", which word is the direct object?
There are two direct objects in the sentence, pump and it.
The noun "pump" is the direct object of the verb "use".
The pronoun "it" is the direct object of the verb "fill".
Note: The adjective "low" is a type of object called a "predicate adjective", an adjective that follows a linking verb to restate the subject of that verb.
A linking verb is a verb that acts as an equal sign, its subject is or becomes the object (tire = low).