Wanafunzi wamo darasani, lakini mwalimu yumo ofisini bado.

Because the verb has to agree with the noun class of the subject.

• wanafunzi = students → plural human noun, so it takes the plural agreement wa-
• mwalimu = teacher → singular human noun, so it takes the singular agreement yu-

Then both forms use -mo, which is a locative element meaning something like in / inside.

So:

• wa- + -mo = wamo → they are in / they are inside
• yu- + -mo = yumo → he/she is in / he/she is inside

This is why the two subjects need different forms.

They are location forms of to be.

In this sentence:

• wamo = they are in there / they are inside
• yumo = he/she is in there / he/she is inside

The important part is -mo, which often gives the idea of being inside a place.

So the sentence is not just saying that the students and teacher exist; it is saying where they are located.

The ending -ni is a very common locative ending in Swahili. It often means:

• in
• at
• to
• sometimes on, depending on context

So:

• darasa = classroom / class
• darasani = in the classroom / in class

and

• ofisi = office
• ofisini = in the office / at the office

This -ni ending is one of the first things learners notice in location expressions.

A very common learner question.

Broadly speaking:

• yuko / wako are more general location forms: is/are there, is/are at
• yumo / wamo often suggest being inside something or being within a place

So:

• Wanafunzi wako darasani = The students are in the classroom
• Wanafunzi wamo darasani = The students are in the classroom / inside the classroom

In many everyday situations, yuko/wako would also sound natural. But yumo/wamo adds a slightly stronger inside/in it feel.

Yes, that would also be correct and natural.

Your original sentence with wamo and yumo is a bit more specific in its locative sense, especially with the idea of being inside those places.

So both are possible:

• wamo / yumo → more inside/in there
• wako / yuko → more general are/is located there

A learner should understand both.

Lakini means but.

It joins two contrasting ideas:

• Wanafunzi wamo darasani = The students are in the classroom
• lakini = but
• mwalimu yumo ofisini bado = the teacher is still in the office

So the contrast is: the students are already in one place, but the teacher is still somewhere else.

Here, bado means still.

So:

• mwalimu yumo ofisini bado = the teacher is still in the office

It shows that the situation has continued longer than expected or longer than some other situation.

Common uses of bado include:

• Bado niko hapa. = I am still here.
• Bado hajafika. = He/she has not arrived yet.
• Bado wanasoma. = They are still studying.

In your sentence, it implies that the teacher has not yet come out of the office or has not yet joined the students.

That is a very good question, because learners often expect all singular human nouns to behave the same way.

With normal verb agreement, a singular human noun usually does take a-:

• mwalimu anasoma = the teacher is studying

But with these special locative forms, the language often uses forms like:

• yuko
• yumo
• yupo

So instead of a-, you get yu- in these location expressions.

That is why:

• mwalimu yumo ofisini not
• mwalimu amo ofisini

For learners, it is best to memorize these as common location forms.

It can mean either, depending on context.

• darasa can refer to a class or classroom
• darasani can therefore mean in class or in the classroom

In your sentence, because it contrasts with ofisini, many learners will naturally understand it as a physical place: in the classroom.

But in other contexts, darasani can also mean in class in the more general sense of attending lessons.

The given order is very natural:

• Wanafunzi wamo darasani, lakini mwalimu yumo ofisini bado.

Swahili usually prefers a straightforward order like:

subject + verb + place

So:

• Wanafunzi wamo darasani
• mwalimu yumo ofisini

You can move some parts for emphasis, especially bado, but the original order is simple and standard for learners.

For example:

• Mwalimu bado yumo ofisini can also be understood, with more emphasis on still

But the version you have is very natural and easy to follow.

Usually:

• mwanafunzi = student / pupil
• wanafunzi = students / pupils
• mwalimu = teacher
• plural: walimu = teachers

In many contexts, wanafunzi can refer to school pupils or students more generally. English may translate it as students or pupils depending on the situation.

So your sentence could be understood as:

• The students are in the classroom, but the teacher is still in the office.

or in some contexts:

• The pupils are in class, but the teacher is still in the office.

A helpful breakdown is:

• Wanafunzi = the students
• wamo = are in / are inside
• darasani = in the classroom

then

• lakini = but

then

• mwalimu = the teacher
• yumo = is in / is inside
• ofisini = in the office
• bado = still

So the whole sentence is built as two simple location statements joined by lakini.

A learner-friendly way to see it is:

[subject] + [location verb] + [place], lakini [subject] + [location verb] + [place] + bado

That makes it easier to create similar sentences of your own.