Onto and on are sometimes used interchangeably, but onto more strongly suggests movement toward something."The dog jumped on the table" may mean that he was already on the table, jumping."The dog jumped onto the table" clearly indicates that he leaped to the table from somewhere else. In constructions where on is an adverb and to a preposition, write and pronounce them as separate words: "We then moved on to the next building."
Dictionary of problem words and expressions. Harry Shaw. 1975.