Nitakupigia simu mara moja nikifika nyumbani.

It breaks down like this:

• ni- = I
• -ta- = future marker, will
• -ku- = you (singular)
• -pigia = call/ring for

So Nitakupigia means I will call you.

The verb root -piga literally has meanings like hit/strike, but in Swahili it is also used in the expression kupiga simu, which means to make a phone call.

The form -pigia is an applicative form. In simple terms, it helps show who the action is directed to or done for. So:

• kupiga simu = to make a phone call
• kum/pigia simu = to call him/her
• kukupigia simu / nitakupigia simu = to call you / I will call you

So Nitakupigia simu is a very natural way to say I will call you.

Simu can mean telephone/phone, and in expressions like kupiga simu or kupigia simu, it refers to a phone call.

So in this sentence, simu is part of the fixed expression for calling someone by phone.

Literally, mara moja means one time. But very often in real usage it means immediately, right away, or at once.

In this sentence, it gives the idea of:

• right away
• immediately
• as soon as

So it adds urgency or promptness to the promise to call.

Nikifika breaks down as:

• ni- = I
• -ki- = when/if
• -fika = arrive

So nikifika literally means when I arrive or if I arrive.

In this sentence, the natural meaning is when I arrive.

Yes. The -ki- form can often mean if or when, depending on context.

Here, because the sentence is talking about a normal future plan, the best reading is when I arrive. It does not usually sound doubtful here. It sounds like a normal statement of intention: I’ll call you when I get home.

Nyumbani comes from nyumba meaning house/home plus the locative ending -ni, which gives a location sense.

So nyumbani means something like:

• at home
• home

Swahili often uses locative forms instead of separate prepositions the way English does. After fika (arrive/reach), nyumbani works very naturally:

• kufika nyumbani = to arrive home

No, it is not completely fixed. The sentence you have is very natural:

• Nitakupigia simu mara moja nikifika nyumbani.

But you could also say:

• Nikifika nyumbani, nitakupigia simu mara moja.

Both are natural. The difference is mainly emphasis:

• original version: starts with the promise I will call you
• reordered version: starts with the time clause when I get home

Yes. You could say:

• Nitakupigia simu mara moja nitakapofika nyumbani.

This is also correct. Nitakapofika is a more explicitly future when I arrive form. It can sound a little more formal or more carefully specified.

By contrast, nikifika is very common, natural, and conversational.

So:

• nikifika nyumbani = very common, natural when I get home
• nitakapofika nyumbani = also correct, a bit more explicit/formal

Usually, -ku- is for you singular.

If you are talking to multiple people, you would normally use -wa-:

• Nitawapigia simu mara moja nikifika nyumbani.

That means I will call you all as soon as I get home.

One thing to notice: -wa- can also mean them, so context is important. But in conversation, the meaning is usually clear.