"Hybrid" move

hi guys, i'm afraid that is again a beginner question : (and maybe an old one too!)

around tactical move :
what if a unity (let's say a solid one like 4Bd for example)

is it possible for the same turn to move part in good going and enter in a bad going terrain?

the 4Bd is for example at less that  1 LB from a Forest and wanted to enter it,
is it possible in the same turn?
and how much move max in the forest?

or is that take 2 turns, one to go near the forest, and the next turn to enter?

and what about the situation that the unity is in the forest, but not on the side and wanted to exit and continue his move in a good going?


a 4Bd can move max 1 Lb in bad going and max 2 in good going,
so we can imagine to have the possibility to move in the 2 terrains if, unit have move less than 1/2 "max move" in good going, so it have 1/2 "max move" in Bad going?
maybe it's a trivial question, but it happens sometimes, especially when trying to climb rocky hills, and with LH unit too...
(imagine LH at 1/2 Lb form the hill side, and forced to move only 1/2 LB and wait next turn to climb the rocky hill?)

thanks,