Gharama zisizo za lazima zimepungua.

Here is a simple breakdown:

• Gharama = costs / expenses
• zisizo za lazima = that are not necessary / unnecessary
• zimepungua = have decreased / have gone down

So the structure is basically:

• noun
  • modifier
    • verb

A very literal reading is:

• Costs that are not necessary have decreased.

More natural English is:

• Unnecessary costs have decreased.

Because zisizo has to agree with the noun gharama.

In Swahili, words that describe or connect to a noun often change form depending on the noun class. Gharama belongs to the N class and here it is being treated as plural (class 10), so the relative form is zisizo.

So:

• gharama ... zisizo ... = costs ... which are not ...

If the agreement were different, the form would change too.

Here it is plural.

That is why you see:

• zisizo instead of a singular form
• zimepungua instead of imepungua

A useful thing to know is that gharama often has the same form in singular and plural, so you tell the difference from the agreement words around it.

In this sentence, the agreement clearly shows plural:

• zisizo
• zimepungua

So the meaning is costs / expenses, not just a cost.

It literally means something like:

• which are not of necessity

But in normal English, that becomes:

• which are not necessary
• unnecessary

So:

• gharama zisizo za lazima = unnecessary costs

This is a very common Swahili way to describe something by using a relative expression rather than a single adjective.

Za is a connector often translated as of.

So:

• za lazima = of necessity

Together with zisizo, it gives the idea:

• zisizo za lazima = not of necessity = not necessary

The form za also agrees with gharama in class 10.

You can think of it as part of a set of agreement forms:

• ya
• wa
• za
• la
• etc.

Which one you use depends on the noun class.

Because sio is not the right form here.

In this sentence, not appears inside a phrase modifying the noun gharama. Swahili usually uses a negative relative form for that, and here that form is zisizo.

So:

• zisizo za lazima = which are not necessary

By contrast, sio is more like it is not / it is not a... in other sentence types.

For example, you might say:

• Hii sio lazima = This is not necessary

But when directly modifying a noun, Swahili prefers the agreeing relative form:

• gharama zisizo za lazima

Zimepungua uses the -me- tense/aspect marker, which often corresponds to the present perfect in English:

• have decreased
• have gone down

Its parts are:

• zi- = subject agreement for class 10 plural
• -me- = perfect/completed action
• -pungua = decrease / become less

So zimepungua literally means:

• they have decreased

Depending on context, English might translate it as:

• have decreased
• decreased
• have gone down

Because the subject is being treated as plural.

With gharama in this sentence, the agreement is class 10 plural, so the verb begins with zi-:

• gharama zimepungua = costs have decreased

If it were singular agreement, you would expect:

• gharama imepungua = cost has decreased

So the verb form helps you see how the speaker is treating gharama.

Here it is intransitive.

That means the subject itself is changing:

• gharama zimepungua = costs have decreased / gone down

No direct object is needed.

If you wanted a transitive idea like reduce costs, Swahili would usually use a different form, such as a causative verb:

• kupunguza gharama = to reduce costs

So:

• gharama zimepungua = the costs decreased
• amepunguza gharama = he/she has reduced the costs

Because in Swahili, modifiers usually come after the noun they describe.

So instead of English-style:

• unnecessary costs

Swahili naturally says:

• gharama zisizo za lazima

This is very normal in Swahili. Noun first, description after.

You would change the agreement to singular.

A likely singular version would be:

• Gharama isiyo ya lazima imepungua.

That means:

• An unnecessary cost has decreased.
• or A non-essential expense has decreased.

Compare:

• isiyo = singular agreement
• ya = singular connector
• imepungua = singular verb

Versus the plural sentence you were given:

• zisizo
• za
• zimepungua

Sometimes Swahili uses full descriptive phrases where English uses a single adjective. In this sentence, zisizo za lazima is already a very natural way to say unnecessary / non-essential.

A learner should get used to this pattern, because Swahili often says things in a more structural way:

• of importance
• not of necessity
• with...
• which...

So even if it feels longer than English, it is normal Swahili.