Li alportas naskiĝateston pro tio, ke la legitimilo eksvalidiĝis.

Because it is the direct object of alportas. In Esperanto, the direct object usually takes the accusative ending -n.

• Li alportas naskiĝateston. = He brings a birth certificate.
• The thing being brought is naskiĝatesto, so it becomes naskiĝateston.

Esperanto very often makes compound words by putting roots together. Here:

• naskiĝ- = birth
• atest- = attestation, certificate
• -o = noun ending

So naskiĝatesto literally means something like birth-attestation, i.e. birth certificate. In Esperanto, this kind of compound is extremely common and natural.

Yes, it would be understandable, because naskiĝa means related to birth. But naskiĝatesto is the more compact and more lexicalized expression, like a standard name for the document.

A useful rule of thumb is:

• compound noun = often feels more like a fixed term
• adjective + noun = often feels a bit more descriptive

Porti means to carry. Adding al- gives the idea of movement to someone or somewhere, so alporti means to bring.

So:

• porti = carry
• alporti = bring

In this sentence, alportas is used because he is bringing the document to a place or person.

Yes, ĉar would also work. Both can mean because.

• Li alportas naskiĝateston, ĉar la legitimilo eksvalidiĝis.
• Li alportas naskiĝateston pro tio, ke la legitimilo eksvalidiĝis.

The expression pro tio, ke is a bit more explicit and literally means something like because of the fact that. It can sound slightly more formal or emphatic than plain ĉar.

In Esperanto, commas are commonly used to separate subordinate clauses, and this is especially normal with ke. In pro tio, ke, the ke clause explains what that tio refers to.

So the comma helps mark the structure:

• pro tio = because of that
• ke la legitimilo eksvalidiĝis = that the ID expired

This punctuation is very standard.

Because la legitimilo refers to a specific, identifiable document: the ID/document. The speaker assumes the listener knows which one is meant.

But naskiĝateston has no la, so it is indefinite: a birth certificate. Esperanto has no separate word for a/an, so the absence of la often does that job.

Legitimilo means an identity or legitimizing document, often something like an ID card or identification document.

The suffix -il- usually means tool, instrument, or means for doing something. So a legitimilo is literally something used to legitimize or prove identity/status.

That may sound a little abstract in English, but in Esperanto it is a normal way to build a word for a document of identification.

No. It is eks-, not ek-.

That is an important difference:

• ek- = begin suddenly, start doing
• eks- = former, no longer

So eksvalidiĝis means became no longer valid, in other words expired. This is a very useful word for documents, permits, passports, cards, and similar things.

Because the two actions happen at different times.

• alportas = he is bringing / brings it now
• eksvalidiĝis = the ID expired earlier

So the sentence means that his current action is caused by an earlier event. Esperanto handles that very straightforwardly by using the tense that matches each part of the situation.