When I put it in D gear and release the brake, sometimes it moves forward, and other times I have to press the accelerator for it to move.

Is the auto hold feature turned on? Just turn off the auto hold feature.

Remember to gently tap to release the auto hold, and be careful to press the pedal deeply to prevent the car from lurching forward.

The pressure you apply to the brake pedal affects the automatic parking system in the car. When the green "P" symbol appears on the dashboard, it means the brake is engaged. If it doesn't show, it means the brake isn't engaged, and the automatic parking isn't activated. Check the manual for more details!

You have activated the auto hold, right? When you're in Drive (D) and press the brake fully, it will engage the auto hold. At this point, releasing the gas pedal won't make the car move. You just need to press the gas pedal again, and it will automatically release the hold and move forward. This feature is useful for when you're waiting at a red light.