Instead of imaging how the particles will move and guess their motion, there is a technique which can help you to determine that.
In addition, when particles are in phase, it means that both particles on the wave have the same velocity and same displacement. It means both have the same speed in the same direction and same distance and direction away from the rest position.
Particles that are out of phase means both are having the same speed but in opposite direction.
Refer to the video below to find out more.
Question 01: 2005/2008 PP P1 Q20/Q18
Question 02: 2018 PP P1 Q24