It’s easy to pass over this one in conversation, but when editing, you may find yourself coming to a complete halt to decide between the two. According to Merriam-Webster, “onto” is a preposition meaning “to a position on” (he hopped onto the bike). Separately, “on” is an adverb and “to” is a preposition, which refer to position and movement respectively (he hung on to the handlebars).
For me, those definitions are too similar to quickly make a distinction. It helps to determine whether “on” is part of the verb it’s modifying (hopped on), but fortunately, The Chicago Manual of Style offers a more useful trick: mentally say “up” before “on.” If the sentence still makes sense, then “onto” is probably the right choice (he hopped up onto the bike).
