modal verb points the the fact that one another representation is
constructed. The participant is  detached from the current activity. In this
case other properties of the situational level are considered and the modal
verb marks the passage between enunciative and situational level .
For example in trial 2 the modal verb 'can' appears for the move of disk 2,
planned in 1. In trial 1, there was a sitational location on the peg in 2-3
: move of disk 1 was located to move of disk 2 for regrouping set of disks