Nilipofungua sanduku, niliona barua yako.

It has several parts:

• ni- = I
• -li- = past tense
• -po- = when / at the time when
• fungua = open

So nilipofungua means when I opened.

Because in Swahili, a lot of information is built directly into the verb.

A single verb word can contain:

• the subject
• the tense
• a relative or time marker
• sometimes an object marker
• the verb root itself

So where English uses several words, Swahili often uses one long verb form.

Here, -po- gives the idea of when or at the time that.

Compare:

• nilifungua sanduku = I opened the box
• nilipofungua sanduku = when I opened the box

So -po- changes the clause from a simple statement into a time-setting clause.

Because niliona is the main action of the sentence: I saw.

The first clause, Nilipofungua sanduku, only gives the time background:

• When I opened the box...

The second clause gives the main event:

• ...I saw your letter.

So:

• nilipofungua = background time clause
• niliona = main past action

Because ni- already means I.

Swahili usually does not need a separate subject pronoun unless you want emphasis.

So:

• niliona = I saw
• mimi niliona = I saw / I was the one who saw, with extra emphasis

Swahili does not have articles like English a, an, or the.

That means:

• sanduku can mean a box or the box
• barua can mean a letter or the letter

The exact meaning depends on context.

It literally means letter your, but in natural English that becomes your letter.

The parts are:

• barua = letter
• yako = your

Swahili usually puts the possessive after the noun, not before it.

So:

• barua yako = your letter
• compare English: your letter

Because Swahili possessives agree with the noun class of the noun they describe.

Barua takes the agreement form that gives yako for your.

So learners often memorize this as a pattern:

• barua yako = your letter

You do not always use the same possessive form with every noun:

• kitabu chako = your book
• barua yako = your letter

So the beginning of the possessive changes depending on the noun class.

Yes. You could also say:

Niliona barua yako nilipofungua sanduku.

That still means the same thing.

The version with Nilipofungua... first is very natural because it sets the scene before giving the main action, just like English often does with When I opened the box, ...

Its basic meaning is when or at the time that.

In context, it can sometimes feel close to as soon as, especially if the second action happened immediately after the first. But the core idea is still when.

If you want to be more clearly explicit about after, Swahili often uses other structures, such as baada ya.

Because the sentence begins with a subordinate time clause:

• Nilipofungua sanduku = time/background
• niliona barua yako = main statement

This is similar to English punctuation in a sentence like When I opened the box, I saw your letter.

The comma helps show the pause and structure.