La periodo finiĝis hieraŭ.

La is the definite article, meaning the. It shows that the speaker has a specific period in mind, not just any period.

• la periodo = the period
• periodo = a period / period in a general sense

So La periodo finiĝis hieraŭ refers to some known or previously mentioned period.

In Esperanto, -o marks a noun.

So:

• periodo = a noun meaning period
• the root is period-
• -o tells you it is the noun form

This is a very regular feature of Esperanto:

• libro = book
• domo = house
• tempo = time

Finiĝis comes from:

• fin- = end, finish
• -iĝ- = become, come into a state
• -is = past tense

So finiĝis literally means something like became finished or came to an end.

In natural English, that is usually just ended.

This is an important pattern in Esperanto:
-iĝ- often shows that the subject undergoes a change of state.

Examples:

• rompiĝis = broke / got broken
• malfermiĝis = opened
• sidiĝis = sat down / became seated

Because Esperanto often distinguishes between:

• fini = to finish something
• finiĝi = to end / to come to an end

So:

• Mi finis la libron. = I finished the book.
• La kunveno finiĝis. = The meeting ended.

In your sentence, the period is not finishing some other object. It is itself coming to an end, so finiĝis is the natural choice.

No. It is not a passive.

Finiĝis is an intransitive change-of-state verb built with -iĝ-. It means that the subject itself entered the state of being finished.

Compare:

• La periodo finiĝis. = The period ended.
• La periodo estis finita. = The period was finished.

The second one is passive and suggests an outside action more strongly.
The first one simply says that the period came to an end.

-is is the Esperanto past-tense verb ending.

So:

• finiĝas = ends / is ending
• finiĝis = ended
• finiĝos = will end

This ending works the same way for all verbs, which is one of Esperanto’s most regular features.

Because hieraŭ is an adverb meaning yesterday.

Adverbs in Esperanto often end in -e, but some common time words like hieraŭ, hodiaŭ, and morgaŭ are already fixed adverbial words.

So here:

• La periodo finiĝis = The period ended
• hieraŭ = yesterday

Together: The period ended yesterday.

Yes. Esperanto word order is fairly flexible, as long as the sentence remains clear.

All of these are possible:

• La periodo finiĝis hieraŭ.
• Hieraŭ la periodo finiĝis.
• La periodo hieraŭ finiĝis.

The first one is probably the most neutral here.
Putting hieraŭ first gives it a little more emphasis.

Because there is no direct object here.

In Esperanto, -n usually marks the direct object.

For example:

• Mi finis la periodon. = I finished the period.

Here, la periodon has -n because it is the object of finis.

But in La periodo finiĝis hieraŭ, la periodo is the subject, not the object, so it stays periodo, not periodon.

Also, hieraŭ is an adverb, so it does not take -n here.

Often yes, but context matters.

Periodo can mean a span of time, an era, a phase, or a period in a schedule, depending on context. It is a close cognate to English period, so it is usually easy to understand.

Still, learners should remember that cognates are not always identical in every context. The exact meaning depends on what kind of period is being discussed.

Esperanto stress is very regular: it always falls on the second-to-last syllable.

So:

• La = lah
• pe-ri-O-do
• fi-ni-ĜIS
• hi-e-RAŬ

A rough pronunciation guide:

lah peh-ree-OH-doh fee-nee-JEES hee-eh-ROW

The important stress points are:

• periOdo
• finiĜIS
• hieraŭ has stress on raŭ

This regular stress system makes Esperanto pronunciation much easier than English spelling and stress.