Karne iliyopita ilikuwa muhimu.

A simple breakdown is:

• karne = century
• iliyopita = that passed / past / previous
• ilikuwa = was
• muhimu = important

So the structure is basically:

noun + relative description + past form of "to be" + adjective

Literally, it is something like:

century that-passed it-was important

Natural English turns that into The past century was important.

Iliyopita literally means that passed or which passed.

It comes from the verb -pita, which means to pass. In this sentence, it describes karne:

• karne iliyopita = the century that passed
• more natural English: the past century or last century

So this is a very common Swahili way to say past, previous, or last with time words.

Because they both agree with karne, which belongs to noun class 9 in the singular.

In Swahili, words connected to a noun often show agreement with that noun. Here:

• i-li-yo-pita = class 9 agreement + past + relative marker + pass
• i-li-kuwa = class 9 agreement + past + be

So the i- is not random. It shows that both forms are linked to karne.

Ilikuwa is built from the verb -kuwa, meaning to be or to become.

Its parts are:

• i- = subject marker agreeing with karne
• -li- = past tense
• -kuwa = be

So:

• ilikuwa = it was

Since karne is the subject, English translates it as was in this sentence:

• Karne iliyopita ilikuwa muhimu = The past century was important

Swahili does not have articles like English the and a/an.

That means karne can mean:

• a century
• the century

The exact meaning depends on context. In karne iliyopita, the phrase is normally understood as the past century or last century, even though there is no separate word meaning the.

Usually it means the past century or last century, not a century ago.

That is an important distinction:

• karne iliyopita = the previous/past century
• a century ago would usually be expressed differently, for example with a phrase meaning one hundred years ago

So in this sentence, the idea is about a whole century in the past, not one point in time a hundred years ago.

Many Swahili adjectives change to agree with the noun class, but muhimu is one of the adjectives that commonly stays the same in form.

So you get:

• karne muhimu = an important century
• miaka muhimu = important years
• jambo muhimu = an important matter

In this sentence, muhimu simply means important and does not need to change form.

Yes, but it would change the tense.

• Karne iliyopita ilikuwa muhimu = The past century was important
• Karne iliyopita ni muhimu = The past century is important

So ilikuwa makes the sentence clearly past tense. If you use ni, you are making a present-time statement about the past century.

You would change the agreement to plural:

• Karne zilizopita zilikuwa muhimu = The past centuries were important

Notice the changes:

• iliyopita → zilizopita
• ilikuwa → zilikuwa

This is because the plural agreement for this noun is different, even though karne itself often stays the same in form.

This is a very useful distinction:

• iliyopita = that passed / which passed
  This is a relative form describing a noun.
• ilipita = it passed
  This is a normal main verb.

So:

• karne iliyopita = the century that passed
• karne ilipita = the century passed

In your sentence, iliyopita is not the main verb of the sentence. It is part of the description of karne. The main verb is ilikuwa.