Asilimia ya wanafunzi waliochelewa leo ni ndogo.

Asilimia means percentage or proportion here.

So Asilimia ya wanafunzi waliochelewa leo means the percentage of students who were late today.

A useful extra point:

• asilimia kumi = ten percent
• asilimia ya wanafunzi = the percentage of the students

So Swahili asilimia can correspond to both percent and percentage, depending on the structure.

Ya means of in this sentence.

The pattern is:

noun + ya + noun

So:

• asilimia ya wanafunzi = percentage of students

More specifically, ya is the possessive/genitive connector that agrees with asilimia. Native English speakers often just need to remember that this is the normal way to say of between nouns in Swahili.

Because wanafunzi is plural.

• mwanafunzi = student
• wanafunzi = students

Since the sentence talks about a group of students, the plural form wanafunzi is needed.

Waliochelewa means who were late.

It comes from the verb -chelewa, which means to be late.

So:

• walichelewa = they were late
• waliochelewa = who were late / those who were late

In this sentence, it describes wanafunzi:

• wanafunzi waliochelewa leo = students who were late today

It can be broken down like this:

• wa- = plural subject marker for people, they
• -li- = past tense
• -o- = relative marker, giving the meaning who
• chelewa = be late

So the whole form means who were late.

You do not need to memorize every piece immediately, but it helps to know that this is a very common Swahili way to build who/that/which clauses directly into the verb.

Because walichelewa means they were late, while waliochelewa means who were late.

That difference is important.

Compare:

• wanafunzi walichelewa leo = the students were late today
• wanafunzi waliochelewa leo = the students who were late today

In your sentence, the speaker is not making students the whole statement. They are talking about the percentage of the students who were late. So a relative form, waliochelewa, is needed.

Yes. That is a perfectly normal longer version.

• wanafunzi waliochelewa leo
• wanafunzi ambao walichelewa leo

Both mean students who were late today.

The version in your sentence is more compact and very common in Swahili. Learners often first meet the ambao version because it is easier to recognize, but the shorter relative form is extremely important to get used to.

Leo means today, and here it modifies the idea of being late.

So:

• wanafunzi waliochelewa leo = students who were late today

It does not mean that the percentage exists today in some separate way; it specifically tells you when the lateness happened.

Ni is the copula here, meaning is/are.

It links the subject and the description:

• Asilimia ya wanafunzi waliochelewa leo = subject
• ni ndogo = is small/low

So the sentence structure is basically:

The percentage of students who were late today + is + low

Because ndogo describes asilimia, not wanafunzi.

This is a very important point. The sentence is saying:

• The percentage is low

It is not saying:

• The students are small
• The late students are few in a directly grammatical sense

The head noun of the whole subject is asilimia, so the adjective agrees with that noun, not with wanafunzi inside the of phrase.

Yes, ndogo literally means small or little.

But in English, when we talk about numbers, rates, or percentages, we often say low. So in this sentence:

• ni ndogo literally = is small
• natural English = is low

That is a normal translation choice, not a change in meaning.

Swahili does not have articles like the or a/an.

So a phrase like asilimia ya wanafunzi can be interpreted from context as:

• the percentage of the students
• a percentage of students
• the proportion of students

English needs an article, but Swahili usually does not. The exact English wording depends on context and what sounds most natural.

Not exactly.

Your sentence specifically talks about a percentage or proportion:

• The percentage of students who were late today is low

That is close in meaning to Few students were late today, but it is not identical.

Why not?

• Few students were late today focuses on the number of students
• The percentage ... is low focuses on the proportion of the whole group

If the total number of students matters, percentage is the more precise idea.