In principio liberi tacent.

Because it follows the preposition in in a location/time sense.

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.

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.

Here liberi is nominative plural masculine, and it is the subject of tacent.

It comes from liber meaning free as an adjective, but liberi can also be used as a noun meaning free people or children, depending on context. Since you said the meaning is already known to the learner, the important grammatical point is that it is a plural subject.

Because liberi is plural, the verb is plural too: tacent.

Yes. Liberi is a form that can be ambiguous.

It can be:

• the plural of the adjective liber, libera, liberum = free
• a noun meaning children
• sometimes a substantive meaning free men / free people

A learner often has to rely on context to know which meaning is intended. Latin does this a lot: the same form may have several possible meanings, and context tells you which one fits.

Because the subject is plural.

Latin present tense endings help you identify the subject:

• -t = he/she/it
• -nt = they

So:

• tacet = he/she/it is silent or keeps silent
• tacent = they are silent or keep silent

Since liberi is plural, tacent is the correct form.

Tacent is present tense, third person plural, active voice.

It comes from the verb taceo, tacere, meaning to be silent or to keep silent.

So grammatically it means they are silent or they keep silent.

Yes. In Latin, many verbs that describe a state are still grammatically active.

So even though English often uses to be plus an adjective, Latin may use a normal active verb:

• taceo = I am silent / I keep silent
• tacent = they are silent / they keep silent

This is perfectly normal Latin usage.

Because Latin word order is much freer than English word order.

English depends heavily on order:

• The children are silent
• Are silent the children sounds odd

Latin, however, shows grammatical function mainly through endings, not position. So liberi is recognized as the subject because of its form, and tacent is recognized as the verb because of its ending.

That means Latin can arrange words for emphasis, rhythm, or style.
In principio liberi tacent is perfectly normal, even though English would usually put the subject first.

Yes, to a large extent.

For example, these would still be grammatically understandable Latin:

• Liberi in principio tacent
• Tacent liberi in principio
• In principio tacent liberi

The basic meaning stays the same because the endings still show what each word is doing.

However, the emphasis can shift:

• starting with in principio highlights the time setting
• starting with liberi highlights the subject
• starting with tacent highlights the action

It is a common Latin way to say at the beginning or in the beginning.

This is an ablative phrase of time or circumstance. Latin often uses prepositional phrases like this to place an action within a time frame.

So the phrase answers the question When?
When do they keep silent? In principio.

It is singular.

The dictionary form is principium, a neuter noun of the second declension. Its ablative singular is principio.

Its plural forms would be different, for example:

• nominative/accusative plural: principia
• ablative plural: principiis

So in principio specifically means in/at the beginning, singular.

No. Latin usually does not need an explicit subject pronoun if the verb ending already shows the person and number.

The ending -nt in tacent already means they. So Latin can simply say tacent without writing ei or another pronoun.

Here liberi is present because it names who the they are. But Latin does not need a separate word meaning they in addition to that.