Without a parable spake he not unto them - That is, the things pertaining to his kingdom. On other subjects he spake without parables. On these, such was their prejudice, so many notions had they contrary to the nature of his kingdom, and so liable would plain instructions have been to give offence, that he employed this method to “insinuate” truth gradually into their minds, and to prepare them fully to understand the nature of his kingdom.
They were alone - His disciples.
He expounded - Explained. Showed them more at length the spiritual meaning of the parables.