How should I check if the charactercontroller is grounded next in the next calculation?

I need to find out whether my charactercontroller is grounded or in the next calculation, but I have no idea how I should do that.

If you mean you need to determine if the player will be underground in the next frame, you can use Extrapolation. You would take the character’s current downward velocity (and acceleration) and the last frame time (or frame time average) and forward extrapolate the position of the character assuming the next frame will occur at the same interval. In other words, take the current position and add velocity * deltaTime + accel * deltaTime * deltaTime / 2. Then you can look at the bounds.extents property of the collider to determine if they go under ground. You can use Terrain.SampleHeight() if you are using the built-in terrain to get the terrain height at the player’s location.