Kwenye ubao, mstari mmoja ni wima na mwingine ni mlalo.

Kwenye here means something like on, in, or at, depending on context. In this sentence, kwenye ubao is best understood as on the board.

It is a very common location word in Swahili. With surfaces like a board, paper, or wall, English often uses on, while Swahili often uses kwenye.

So:

• kwenye ubao = on the board

Swahili often puts a place expression first when it sets the scene.

So:

• Kwenye ubao, mstari mmoja ni wima na mwingine ni mlalo.

is like saying:

• On the board, one line is vertical and another is horizontal.

This fronted location is natural and helps establish where everything is. You could also say the sentence without starting with kwenye ubao, but putting it first sounds very normal.

Ubao usually means board. Depending on context, it could be:

• a blackboard / whiteboard
• a plank / piece of wood
• a board as a flat surface

In this sentence, it almost certainly means a classroom-style board or some surface with lines on it.

Because mmoja agrees with the noun class of mstari.

Mstari belongs to a noun class that takes m- / mw- agreement in the singular, so one line is:

• mstari mmoja

not:

• mstari moja

A useful thing to remember is that in Swahili, words like one often change form to match the noun they describe.

Mmoja means one.

So:

• mstari mmoja = one line

It is not just the dictionary form of one copied directly; it is the form that matches mstari.

Because Swahili often leaves out a noun when it is already clear from the context.

So after saying:

• mstari mmoja = one line

the sentence can simply continue with:

• na mwingine = and another / and the other one

Here, mwingine stands in for mstari mwingine because mstari has already been mentioned.

So the full idea is:

• mstari mmoja ni wima na (mstari) mwingine ni mlalo

It can mean either, depending on context.

In this sentence, English could naturally translate it as:

• another
• the other one

Because there are two lines being contrasted, the other one often feels especially natural in English. But the Swahili form mwingine itself does not force only one of those translations.

For the same reason as mmoja: agreement.

Since the missing noun is mstari, the word -ingine takes the matching class prefix and becomes:

• mwingine

So:

• mstari mwingine = another line / the other line

Even when mstari is omitted, the agreement remains on mwingine.

Ni is the copula here, basically the equivalent of is / are in English.

So:

• mstari mmoja ni wima = one line is vertical
• mwingine ni mlalo = the other is horizontal

A useful point: ni does not change form here. It stays ni.

They are descriptive words meaning:

• wima = vertical
• mlalo = horizontal

In this sentence, they come after ni, so they function like predicate descriptions:

• ni wima = is vertical
• ni mlalo = is horizontal

You can think of them as the words giving the orientation of the lines.

In this kind of sentence, after ni, these words are commonly used in a fixed form.

So you say:

• mstari mmoja ni wima
• mwingine ni mlalo

without changing wima or mlalo to match mstari.

For a learner, the safest thing is to memorize these exactly as used:

• ni wima = is vertical
• ni mlalo = is horizontal

Swahili does not normally use articles like English a, an, and the.

That means a phrase like:

• mstari mmoja

can mean:

• one line
• sometimes, depending on context, something like a certain one line

And mwingine can be understood as:

• another
• the other one

English has to choose an article, but Swahili usually does not mark that in the same way.

The comma reflects a pause and helps show that kwenye ubao is setting the scene:

• On the board, ...

It is useful in writing, but the main point is the structure, not the punctuation itself. In speech, you would usually hear a slight pause there.

Yes. You could say:

• Kwenye ubao, mstari mmoja ni wima na mstari mwingine ni mlalo.

That is completely understandable. The version without the second mstari is just more natural and less repetitive.

So both work, but:

• ... na mwingine ni mlalo

sounds smoother.

A useful breakdown is:

• Kwenye ubao = location
• mstari mmoja = subject 1
• ni wima = is vertical
• na = and
• mwingine = the other one / another
• ni mlalo = is horizontal

So the overall pattern is:

• [location], [one noun] is X and [the other] is Y

That is a very common and useful Swahili sentence pattern.