Asha atanunua simkadi mpya kesho kwa sababu ya darasa la mtandaoni.

atanunua can be broken down like this:

• a- = he/she
• -ta- = future tense (will)
• -nunua = buy

So atanunua means he/she will buy.

In this sentence, because the subject is Asha, it means Asha will buy.

In Swahili, the verb normally includes a subject marker, even when the subject noun is stated separately.

So:

• Asha atanunua = Asha will buy
• Atanunua = She will buy / He will buy (if the person is already understood from context)

This is normal Swahili grammar. English usually does not repeat the subject inside the verb, but Swahili does.

In Swahili, adjectives usually come after the noun they describe.

So:

• simkadi mpya = new SIM card
• literally: SIM card new

This is the usual word order in Swahili:

• kitabu kikubwa = big book
• nyumba nzuri = nice house

Yes, the adjective does agree with the noun, but here the form happens to stay mpya.

simkadi is treated like a loanword in the N-class (often class 9/10). Many nouns in this class do not visibly change in the singular and plural, and adjectives often take forms like:

• mpya = new
• nzuri = good/nice
• kubwa = big

So simkadi mpya is the correct way to say new SIM card.

kwa sababu ya means because of.

It is a fixed expression:

• kwa = by/with/for
• sababu = reason
• ya = of

So literally it is something like for the reason of, but naturally in English we translate it as because of.

In this sentence:

• kwa sababu ya darasa la mtandaoni = because of an online class

The ya links sababu (reason) to the thing causing that reason.

So:

• kwa sababu ya mvua = because of the rain
• kwa sababu ya kazi = because of work
• kwa sababu ya darasa la mtandaoni = because of an online class

If a full clause comes after because, Swahili often uses kwa sababu without ya:

• kwa sababu Asha ana darasa = because Asha has a class

So ya is especially natural when it is followed by a noun phrase.

The connector la agrees with darasa.

• darasa belongs to noun class 5 in the singular
• class 5 uses la for of

So:

• darasa la mtandaoni = class of online/internet → online class

If it were plural, the connector would change:

• madarasa ya mtandaoni = online classes

So the choice of la is about noun-class agreement.

mtandaoni means online or on the internet.

It comes from:

• mtandao = network / internet
• -ni = a locative ending, often meaning in/at/on

So mtandaoni literally means something like on the network/internet.

That is why:

• darasa la mtandaoni = online class
• mkutano wa mtandaoni = online meeting

kesho means tomorrow, and its position is fairly flexible.

The given sentence:

• Asha atanunua simkadi mpya kesho...

is perfectly natural.

You could also say:

• Kesho Asha atanunua simkadi mpya...

This puts more emphasis on tomorrow.

So Swahili often allows time words like kesho, leo, and jana to move around more freely than in English, though some positions sound more natural than others depending on emphasis.

As written, it mainly expresses a reason:

• kwa sababu ya darasa la mtandaoni = because of an online class

So the idea is that the online class is the reason she will buy the SIM card.

If you wanted to make the idea of purpose more explicit, Swahili could use other expressions, such as:

• kwa ajili ya darasa la mtandaoni = for the online class
• ili ahudhurie darasa la mtandaoni = so that she can attend the online class

So kwa sababu ya is best understood here as because of / due to.

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

So:

• simkadi mpya can mean a new SIM card or the new SIM card
• darasa la mtandaoni can mean an online class or the online class

The exact meaning depends on context.

That is very normal in Swahili. Learners often want to translate every English article, but usually Swahili simply does not use one.