If spacecraft moving in a stable speed or acceleration suddenly stopped would an astronaut continue moving forward (such as a man in a train) ?
“Zero gravity” is something that doesn't exist. Maybe you mean the situation of freely orbiting spacecraft, in which cosmonauts don't feel any attraction as they are orbiting together with the spacecraft.
Yes, the astronaut will continue to move forward, with all tragic consequences for him.
In the case of stable or unstable acceleration the astronaut will feel the attraction (in the opposite direction of the acceleration), even if spacecraft don't stop.
Yes, the astronaut will move forward. The Law of Inertia of Newton holds good both in presence or absence of Gravity.