Dada yangu mkubwa ananisaidia kujifunza Kiswahili.

That is a very common question. -kubwa basically means big, great, important, senior. With family words, it often means older/elder rather than physically big.

So:

• dada mkubwa = older sister
• dada mdogo = younger sister

In this sentence, Dada yangu mkubwa means my older sister, not my big sister in the physical sense.

This is one of those forms that learners notice quickly. Some family words such as dada, kaka, mama, baba behave a little unusually.

With dada, the normal possessive form is:

• dada yangu = my sister

But elsewhere with many other nouns referring to people, you often see -wangu, for example:

• mtoto wangu = my child
• mwalimu wangu = my teacher

So the short answer is: dada yangu is simply the normal idiomatic form you should learn as a set phrase.

Because in Swahili, modifiers usually come after the noun, not before it.

So the order is roughly:

• dada = sister
• yangu = my
• mkubwa = older/elder

Literally, the phrase is closer to sister my older.

That feels unusual to an English speaker, but it is normal Swahili word order.

Swahili adjectives usually agree with the noun class of the noun they describe.

Because dada refers to one person, the adjective takes the singular human/animate agreement form here:

• mkubwa = big/older for a singular person

That is why you get:

• dada mkubwa = older sister

not just dada kubwa.

It can be broken down like this:

• a- = he/she
• -na- = present tense / ongoing present
• -ni- = me
• -saidia = help

So:

• a-na-ni-saidia

means he/she is helping me or he/she helps me.

In this sentence, because the subject is dada yangu mkubwa, it means my older sister helps/is helping me.

In Swahili, object pronouns are very often built directly into the verb.

So instead of saying something like she helps me with a separate word for me, Swahili commonly says:

• ananisaidia

where -ni- already means me.

You can use a separate pronoun like mimi, but usually that is for emphasis or contrast, not because it is required.

Ku- is the infinitive marker, the equivalent of to in to learn.

So:

• kujifunza = to learn

After a verb like ananisaidia (she helps me), Swahili uses the infinitive:

• ananisaidia kujifunza Kiswahili
• she helps me to learn Swahili / she helps me learn Swahili

A useful note: although jifunza historically contains a reflexive element, in normal modern usage you can simply learn it as the regular verb to learn / study.

In Swahili itself, the language is called Kiswahili.

The prefix Ki- is part of the noun form used for languages (and some other kinds of nouns). So:

• Kiswahili = the Swahili language

Related forms include:

• Mswahili = a Swahili person
• Waswahili = Swahili people

In English, people often just say Swahili, but in Swahili the normal form is Kiswahili.

It can mean either, depending on context.

The -na- tense marker often covers both:

• a general present: helps me
• a current ongoing present: is helping me

So this sentence could mean:

• My older sister helps me learn Swahili
• My older sister is helping me learn Swahili

Context tells you which one is meant.

Because Swahili generally does not use articles like English a and the.

Instead, definiteness is usually understood from:

• context
• possessives
• demonstratives
• the situation

Here, yangu already makes the noun specific:

• dada yangu = my sister

So no separate word for the is needed.

Then the sentence becomes:

• Dada yangu ananisaidia kujifunza Kiswahili.

That simply means:

• My sister helps/is helping me learn Swahili.

Without mkubwa, you are no longer specifying that she is the older sister.

If you wanted younger sister, you could say:

• dada yangu mdogo = my younger sister