Gharama ya ghafla ilipunguza kiasi kilichobaki.

Ya is the linking word that corresponds to of and must agree with the noun class of gharama.

So:

• gharama = cost/expense
• ya = of for this noun class
• ghafla = suddenness / suddenly / unexpectedly

Literally, gharama ya ghafla is something like a cost of suddenness, but in natural English it means a sudden expense or an unexpected cost.

In Swahili, many ideas that English expresses with an adjective are often expressed with a noun + -a connector.

So instead of directly saying something like sudden cost, Swahili commonly says:

• gharama ya ghafla

This is a very normal pattern. Compare similar structures like:

• mtu wa maana = an important person
• siku ya mwisho = the last day
• habari ya kusikitisha = sad news

So ya is not optional here.

Here ghafla gives the idea of sudden, unexpected, or out of nowhere.

On its own, ghafla is often used adverbially:

• Alifika ghafla. = He/She arrived suddenly.

In gharama ya ghafla, it is part of a descriptive phrase meaning sudden expense or unexpected expense.

The i- is the subject marker agreeing with gharama.

Breakdown:

• i- = subject marker for this noun class
• -li- = past tense
• -punguza = reduce, lessen

So ilipunguza = it reduced.

Because gharama is the subject, the verb must agree with it. That is why Swahili uses i- here.

-li- is the normal past tense marker.

So:

• ilipunguza = it reduced
• inapunguza = it is reducing / it reduces
• itapunguza = it will reduce
• imepunguza = it has reduced

In your sentence, ilipunguza places the action in the past.

The base verb is -punguza, which means to reduce, to lessen, or to decrease.

Examples:

• Kupunguza matumizi = to reduce expenses/use
• Kupunguza bei = to lower the price
• Kupunguza kiasi = to reduce the amount

So ilipunguza kiasi means it reduced the amount.

Kiasi means amount, quantity, or sometimes sum.

In this sentence, it refers to the amount that was still left.

So:

• kiasi = amount
• kiasi kilichobaki = the amount that remained / the remaining amount

Depending on context, it could refer to money, supplies, time, or some other measurable quantity.

Kilichobaki is a relative verb form meaning that remained or which was left.

It can be broken down like this:

• ki- = agreement with kiasi
• -li- = past tense
• -cho- = relative marker meaning which/that
• -baki = remain / be left

So kilichobaki literally means which remained.

That is why:

• kiasi kilichobaki = the amount which remained
  or more naturally, the remaining amount

Both are there because the verb is agreeing with kiasi, which belongs to a noun class that uses ki-/cho agreement.

In relative constructions, Swahili often shows agreement more than once inside the verb.

So in kilichobaki, the form tells you that the thing which remained is kiasi.

This agreement is one of the most important features of Swahili grammar: nouns control the form of words around them.

Because it functions like a relative clause modifying kiasi.

English:

• the amount that remained

Swahili:

• kiasi kilichobaki

This is a very common Swahili pattern:

• mtu aliyekuja = the person who came
• kitabu nilichonunua = the book that I bought
• chakula kilichobaki = the food that remained / leftover food

So kilichobaki comes after the noun it describes.

Here it is treated as singular, because the verb uses singular agreement:

• i-lipunguza = it reduced

If it were plural, the agreement would change. For example:

• Gharama za ghafla zilipunguza kiasi kilichobaki.
• Unexpected expenses reduced the remaining amount.

Notice the changes:

• ya → za
• ilipunguza → zilipunguza

So the agreement tells you whether gharama is singular or plural in context.

Yes, depending on context, it can be translated in a few natural ways:

• the amount that remained
• the amount left
• the remaining amount
• sometimes simply what was left

The Swahili is a little more explicit because it names kiasi = amount.

Yes. Even if the meaning is already known, it is useful to notice that several English versions are possible:

• The sudden expense reduced the amount that remained.
• The unexpected cost reduced the remaining amount.
• The surprise expense lowered what was left.

All of these fit the Swahili structure reasonably well. The exact best translation depends on context, especially whether kiasi refers specifically to money.