Noong isang gabi, hindi nakatulog si Liza dahil masakit ang isang tainga niya.

It means one night or one evening.

• Noong introduces a time in the past.
• isang means one or a/an.
• gabi means evening or night, depending on context.

So Noong isang gabi is a natural way to set the scene: One night...

In modern standard Filipino, noong is commonly used for a specific past time expression, like noong Lunes (last Monday / on Monday) or noong bata pa ako (when I was still a child).

Nang has several other uses, so using noong here makes the time meaning clearer. In this sentence, Noong isang gabi clearly means one night.

Isang is the form of isa used before a noun.

You can think of it as:

• isa = one
• isang gabi = one night
• isang tainga = one ear

So isang is just the normal form when isa directly modifies a noun.

It means couldn't sleep or didn't get to sleep.

• hindi = not
• nakatulog = was able to sleep / managed to fall asleep

So hindi nakatulog si Liza is more natural in English as Liza couldn't sleep, not just Liza did not sleep.

This is an important nuance.

• natulog usually means slept
• nakatulog often means managed to sleep, was able to sleep, or fell asleep

Because the sentence says hindi nakatulog, the idea is that Liza was unable to sleep, probably because of the ear pain.

So:

• Hindi natulog si Liza = Liza did not sleep
• Hindi nakatulog si Liza = Liza couldn't sleep

The second one fits this context better.

Si is the personal-name marker used for a singular person’s name.

So:

• si Liza
• si Marco
• si Ana

In this sentence, si Liza marks Liza as the main noun phrase being talked about.

This is a normal part of Filipino grammar and does not translate directly into English.

Dahil means because.

Yes, you could also use kasi in many everyday situations:

• ... dahil masakit ang isang tainga niya
• ... kasi masakit ang isang tainga niya

Both can mean because one of her ears hurt.
Generally:

• dahil sounds a bit more neutral or formal
• kasi sounds more conversational

Yes. Niya can mean his or her.

Filipino third-person singular pronouns do not show gender. So niya can refer to either a male or a female person.

In this sentence, we know it means her because the person is Liza.

Filipino often puts the predicate first.

Here:

• masakit = painful / hurting
• ang isang tainga niya = one of her ears

So the structure is literally something like:

Painful the one ear of hers

But natural English is:

One of her ears hurts

This pattern is very common in Filipino:

• Masakit ang ulo ko = My head hurts
• Masakit ang kamay niya = His/Her hand hurts

Ang is a grammatical marker. It does not simply mean the.

In this sentence, ang marks the noun phrase being described by masakit. So ang isang tainga niya is the thing that is hurting.

This is one of the most important things to remember in Filipino: words like ang, ng, and sa are markers, not direct English-style articles.

That is a very common Filipino pattern.

Short possessive pronouns often come after the noun:

• tainga niya = her ear
• ulo ko = my head
• kamay mo = your hand

So isang tainga niya is a normal way to say one of her ears or her one ear.

You can also find fuller possessive forms before the noun, such as kaniyang tainga, but the version with niya after the noun is very common and natural.

Yes, it strongly suggests that one ear was hurting, not both.

If the sentence only said masakit ang tainga niya, it could simply mean her ear hurts. But adding isang makes the idea of one ear explicit.

So the sentence is specifically pointing to pain in just one ear.

Yes.

Gabi can refer to evening or night, depending on context. Here, because the sentence talks about not being able to sleep, night is the most natural English choice.

So although Noong isang gabi could sometimes be translated as One evening, in this sentence One night fits better.

Sure:

• Noong = at/on, when (past time)
• isang gabi = one night
• hindi = not
• nakatulog = was able to sleep / managed to sleep / fell asleep
• si Liza = Liza
• dahil = because
• masakit = painful / hurting
• ang isang tainga niya = one of her ears

So a very natural English translation is:

One night, Liza couldn't sleep because one of her ears hurt.