posted by rfvv .
Help them carry their walker up the steps onto the bus.
(What is the part of speech of 'onto the bus'? Is it an adjective phrase or an adverbial phrase? Can we use 'on' instead of 'onto'? Thank you for your help.)
That's a prepositional phrase which is modifying the verb.
... and yes, you can use 'on' if you wish.
I should clarify that. You can use 'on' instead of 'onto' but you'll change the meaning of that phrase. Also, it'll become adjectival instead of adverbial.