Barua ninayoandika kwa mama ni fupi.

A helpful breakdown is:

• Barua = letter
• ninayoandika = that I am writing
• kwa mama = to/for mother, to/for mom
• ni = is
• fupi = short

So the structure is:

Barua + ninayoandika kwa mama + ni fupi
= The letter that I am writing to mother is short.

ninayoandika is one verb form made of several parts:

• ni- = I
• -na- = present / ongoing action
• -yo- = relative marker agreeing with barua
• andika = write

So ninayoandika means something like that I am writing.

It is not just a simple verb like ninaandika = I am writing.
It is a relative form: it describes the noun barua.

Because barua belongs to noun class 9 in the singular, and the relative marker must agree with that noun class.

So:

• barua (singular) → -yo-
• barua (plural) → -zo-

That is why you get:

• Barua ninayoandika... = The letter that I am writing...
• Barua ninazoandika... = The letters that I am writing...

Even though barua looks the same in singular and plural, the agreement inside the sentence shows the difference.

Because in Swahili, the idea of that/which is often built directly into the verb.

So instead of using a separate word, Swahili can use a relative marker inside the verb:

• ninayoandika = that I am writing

English often uses a separate relative word, but Swahili often does not need one.

Yes. That is also correct.

Compare:

• Barua ninayoandika kwa mama ni fupi.
• Barua ambayo ninaandika kwa mama ni fupi.

Both mean the same thing.

The first version is more compact because the relative meaning is built into the verb.
The second version uses ambayo, which is a separate relative word meaning which/that for this noun class.

Here kwa mama most naturally means to mother / to mom.

The preposition kwa is very flexible in Swahili. Depending on context, it can mean things like:

• to
• for
• at
• with
• from

In this sentence, because the verb is andika (write), kwa mama is understood as the person the letter is directed to.

If you wanted to say to my mother more explicitly, you could say:

• kwa mama yangu
• or kwa mamangu

Here ni is the copula, the word that works like is/are.

So:

• barua ... ni fupi = the letter ... is short

In Swahili, ni is commonly used to link the subject with a description or identity.

Examples:

• Mti ni mrefu. = The tree is tall.
• Kitabu ni kizuri. = The book is good.
• Barua ni fupi. = The letter is short.

Because Swahili usually puts the subject inside the verb.

In ninayoandika:

• ni- already means I

So a separate pronoun like mimi is not necessary.

You can add mimi only for emphasis:

• Mimi ninayoandika barua...
• or more naturally in a fuller sentence for emphasis

But normally, the verb already tells you who the subject is.

-na- is the present marker. It often gives a meaning like:

• am doing
• do
• am in the process of doing

In this sentence, the most natural English translation is am writing, because the relative clause is describing an ongoing action:

• ninayoandika = that I am writing

So -na- here has a present/progressive sense.

Swahili adjectives often do agree with the noun class, but the form you see depends on the adjective and the noun class.

With barua (class 9 singular), the adjective appears as fupi.

Compare:

• mtoto mfupi = short child
• kitabu kifupi = short book
• barua fupi = short letter

So yes, agreement exists in Swahili, but with barua the form is simply fupi.

You would say:

Barua ninazoandika kwa mama ni fupi.

The important change is:

• singular: ninayoandika
• plural: ninazoandika

That change shows agreement with plural barua.

So:

• Barua ninayoandika kwa mama ni fupi. = The letter that I am writing to mother is short.
• Barua ninazoandika kwa mama ni fupi. = The letters that I am writing to mother are short.

Because barua is the main thing being talked about.

The sentence structure is:

• first, the noun: barua
• then, the relative clause describing which letter: ninayoandika kwa mama
• then, the main statement about it: ni fupi

So the sentence is organized as:

[the letter I am writing to mother] [is short]

That is very natural in Swahili.