Learn Hardcore Latin: In principio liberi tacent. - At the beginning the children are silent.

In Latin, in + ablative often means in, on, at, or during. Here in principio means at the beginning or in the beginning, so principio is ablative singular of principium.

Compare:

in principio = at/in the beginning
in urbem = into the city (in + accusative, motion toward)
in urbe = in the city (in + ablative, location)

So the ablative here shows a setting in time, not motion.

Why does in take the ablative here instead of the accusative?

Because the phrase does not express movement into something. It expresses a state or position in time.

A very common Latin rule is:

in + accusative = motion into / onto / toward
in + ablative = location or position in / on / at

Since in principio means at the beginning, there is no movement involved. It is simply locating the action in time.

What form is liberi?