Constrain is a verbal agreement, or even a law that limits your personal freedoms. Example: With the judge's decision, the criminal would be constrained from any human contact for two weeks in solitary confinement. The policy of this rental car constrains our ability to leave the state of Alabama before Thursday. Restrain is more specific to the physical state of holding someone down. The mental patient had to be restrained using a straight jacket. If you don't restrain you dog at the park, and it bites someone, they can sue you for personal damages!

To restrain is to prevent from doing. Ex: The cop restrained him, when he tried to attack. To constrain is a use of force to action or though of others. Ex: They were constrained to agree with their parents.

David Noll has the best answer but it's not in the answer section so it can't be voted on.

That was a great answer, Thank you so much!

Restrain and constrain can both be abstract or physical. Constraining means to put a boundary on something. Restraining is preventing a movement that would otherwise occur. A train is constrained by its track, but not restrained by it because its tendency is to move forward. A dog is restrained by its leash, and therefore constrained to the radius of the leash. Restraining is pulling or pushing against some movement. Constraining is guiding or enclosing the movement. A cage is a constraint, but a seatbelt is a restraint. A restraint prevents motion in 1 dimension, but constraints prevent motion in more than 1 dimension