Begi langu liliachwa darasani jana.

Because Swahili possessives must agree with the noun class of the thing possessed.

Here, begi is treated as a singular class 5 noun, so my is langu.

• begi langu = my bag
• but plural mabegi yangu = my bags

So -angu is the basic possessive idea my, and the form changes depending on the noun class.

In this sentence, begi is being used as a class 5 singular noun.

You can tell because of the agreement:

• langu matches class 5
• li- at the start of the verb also matches class 5

Its common plural is mabegi, which is class 6.

So the pattern is:

• begi langu liliachwa = singular
• mabegi yangu yaliachwa = plural

They do two different jobs.

liliachwa can be broken down like this:

• li- = subject marker for a class 5 singular noun (it)
• -li- = past tense marker
• -ach- = verb root from kuacha
• -w- = passive marker
• -a = final vowel

So li-li-ach-w-a means something like it-was-left.

That is why you see two lis in a row.

The basic active verb is kuacha, meaning to leave.

From that:

• kuacha = to leave
• kuachwa = to be left / to be left behind

In the sentence, liliachwa is the past form of the passive verb, agreeing with begi.

So the sentence is built from the idea the bag was left.

Yes. It is passive.

The passive idea in English is was left. Swahili shows that with the passive marker -w- inside the verb:

• kuacha = to leave
• kuachwa = to be left

The sentence focuses on the bag, not on the person who left it.

A more active version would be something like:

• Mtu aliacha begi langu darasani jana.
• Someone left my bag in the classroom yesterday.

Darasa means class or classroom.

The ending -ni is a locative ending. It often gives the sense of:

• in
• at
• sometimes to

So:

• darasa = class/classroom
• darasani = in the classroom / at school/in class, depending on context

In this sentence, darasani means in the classroom.

Because Swahili often places time expressions like jana (yesterday) toward the end of the sentence.

So this order is natural:

• Begi langu liliachwa darasani jana.

But Swahili word order is fairly flexible, especially for adverbs of time. You could also hear:

• Jana begi langu liliachwa darasani.

That version puts more emphasis on yesterday.

Because Swahili usually puts tense and voice information inside the verb itself instead of using a separate helping verb like English was.

In liliachwa:

• past time is shown by -li-
• passive meaning is shown by -w-

So English needs two words, was left, but Swahili can express that in one verb form.

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

So a noun can often be translated with or without the, depending on context.

That means:

• darasani could be understood as in the classroom
• and in another context it might feel more like in a classroom

English translations add the when it sounds natural, but Swahili itself does not need a separate word for it.

You would say:

Mabegi yangu yaliachwa darasani jana.

Changes:

• begi → mabegi for the plural
• langu → yangu
• li- → ya- on the verb to agree with the plural noun class

So:

• Begi langu liliachwa darasani jana. = singular
• Mabegi yangu yaliachwa darasani jana. = plural