Barua isipofika leo, nitaenda posta kesho asubuhi.

Because this clause means if it does not arrive, not simply it does not arrive.

In Swahili, the pattern subject prefix + si + po + verb is commonly used for a negative conditional idea:

• i- = subject agreement for barua
• -si- = negative
• -po- = conditional/time marker, often if/when
• fika = arrive

So isipofika means if it does not arrive.

By contrast, haifiki is more like a plain negative statement such as it does not arrive / it isn’t arriving, not a conditional clause.

It can be broken down like this:

• i- = subject prefix for barua
• -si- = negative marker
• -po- = conditional marker, often translated as if or sometimes when
• -fik- = verb root meaning arrive / reach
• -a = final vowel

So the whole form literally builds the idea it-not-if-arrive, which in natural English becomes if it does not arrive.

Because barua belongs to a noun class that takes i- in the singular.

Swahili verbs must agree with the noun class of the subject. So:

• barua = letter
• singular agreement = i-

That is why the verb begins with i- in isipofika.

A useful extra point: barua often has the same form in singular and plural, so agreement helps you tell the difference. If it were plural, you would usually get zi- instead:

• Barua zisipofika leo... = If the letters do not arrive today...

Not always. -po- can often mean if or when, depending on context.

In this sentence, if is the most natural English translation because the speaker is talking about a possible future situation:

• Barua isipofika leo... = If the letter doesn’t arrive today...

In other contexts, the same kind of form can feel closer to when.

Because Swahili does not have articles like a, an, and the.

Whether something is definite or indefinite is usually understood from context. So:

• barua can mean a letter or the letter
• posta can mean post office or the post office

English needs articles, but Swahili normally does not.

Both kinds of expressions are possible, but kwenda posta is a very natural, common way to say go to the post office.

With verbs of motion like kwenda, Swahili often allows the destination to come directly after the verb, especially with common place words or established expressions.

So:

• nitaenda posta = I will go to the post office
• nitaenda kwenye posta = I will go to the post office

The version with kwenye is a bit more explicit about location, but the shorter form is very normal.

Because the first clause already has a conditional meaning built into isipofika.

The second clause is the main result clause, so it uses the regular future marker:

• ni-ta-enda
• ni- = I
• -ta- = future
• enda = go

So the structure is:

• If X happens / does not happen...
• I will do Y

The first clause does not need -ta- because the conditional form already sets up that future possibility.

Because fika means arrive / reach a destination, which fits a letter very well.

• fika = arrive
• kuja = come

A letter typically arrives rather than comes in Swahili wording. So barua isipofika is the natural way to express if the letter doesn’t arrive.

Yes. Swahili word order is fairly flexible with time expressions.

These are all natural:

• Nitaenda posta kesho asubuhi
• Kesho asubuhi nitaenda posta

The important thing is that leo belongs to the first clause and kesho asubuhi belongs to the second clause.

So the original sentence clearly means:

• today = time of possible arrival
• tomorrow morning = time of going to the post office

It means tomorrow morning.

• kesho = tomorrow
• asubuhi = morning

Together they form a time expression. In English we say tomorrow morning, and Swahili commonly says kesho asubuhi.

You would usually change the verb agreement, not the noun form itself:

• Barua zisipofika leo, nitaenda posta kesho asubuhi.

Here zi- is the plural subject prefix for this noun class.

So:

• barua isipofika = if the letter does not arrive
• barua zisipofika = if the letters do not arrive

This is a very common feature of Swahili: sometimes the noun looks the same in singular and plural, and the agreement on the verb shows the number.