See the notes at Matthew 13:10-17. On Mark 4:12, see the notes at John 12:39-40.
When he was alone - That is, separate from the multitude. When he withdrew from the multitude a few followed him for the purpose of more instruction.
Know ye not this parable? - This which is so plain and obvious.
How then will ye know all parables? - Those which are more difficult and obscure. As they were themselves to be “teachers,” it was important that they should be acquainted with the whole system of religion - of much more importance for them at that time than for the mass of the people.