Learn Hardcore Swahili: Mama alinunua dhahabu sokoni. - Mother bought gold at the market.

a- : the subject prefix for 3rd person singular (“she/it”)
-li- : the simple past tense marker
nunua : the verb root meaning “to buy”
Putting them together (a-li-nunua) gives “she bought.”

How would you say “Mama buys gold at the market” instead of “she bought”?

Swap the past marker -li- for the present tense -na-:
“Mama ananunua dhahabu sokoni.”

What is the difference between nunua and kununua?

There is no change in core meaning.

nunua is the verb root (“buy”)
kununua adds the infinitive prefix ku-, giving “to buy.”
You’ll see kununua when you need an infinitive (“I want to buy…”).

Why is it sokoni instead of soko?

soko means “market” (noun class 5). Adding the locative suffix -ni turns it into “at/in the market.” Hence sokoni = “at the market.”

Why isn’t there an article like “the” or “a” before dhahabu?

Swahili doesn’t use indefinite or definite articles. Context tells you whether dhahabu means “gold” in general, “some gold,” or “the gold.”

How do you form the negative: “Mama did not buy gold at the market”?

Negate by replacing a- + -li- with ha- … -ku-:
“Mama hakununua dhahabu sokoni.”
(ha- = negative subject + ku- = negative tense/mood)

How would you turn the original sentence into a question: “Did Mama buy gold at the market?”

You can simply add Je at the beginning (optional) and use the same verb form, letting intonation do the rest:
“Je, Mama alinunua dhahabu sokoni?”
—or—
“Mama alinunua dhahabu sokoni?”

Can you include an object marker for dhahabu in alinunua?

In theory yes (there is an object infix for noun class 9/10), but with dhahabu it’s almost never used. You just say alinunua dhahabu rather than inserting an extra marker.