Learn Hardcore Swahili: Nimeandika anwani yako kwenye barua. - I have written your address on the letter.

nimeandika is the perfect form of kuandika (to write). It breaks down as:
• ni- (1st-person singular subject “I”)
• -me- (perfect aspect “have”)
• andika (write)
Together, nimeandika means “I have written.”

How would I say “I wrote your address on the letter” instead of “I have written…”?

Replace the perfect marker -me- with the simple-past marker -li-:
• Ni- + -li- + andika = niliandika
So you get:
Niliandika anwani yako kwenye barua.
(“I wrote your address on the letter.”)

Why is there no word for “the” or “a” before anwani yako?

Swahili does not use definite or indefinite articles. Context tells you whether a noun is specific or general. Thus you simply say anwani yako for either “your address” or “the address of yours.”

How is possession shown in anwani yako, and why does yako follow the noun?

In Swahili, possessive pronouns follow the noun they modify.
• anwani = “address”
• yako = “your” (2nd-person singular)
Hence anwani yako = “your address.”

What does kwenye mean, and can I use a different word?

kwenye is a preposition meaning “on,” “in,” or “at,” depending on context. Here it means “on” (the surface of) the letter. An alternative is juu ya (literally “on top of”):
• Nimeandika anwani yako juu ya barua.

What does barua refer to? Is it the same as email?

barua by itself means a physical letter (paper mail). An email is specifically barua pepe (“smoke‐letter,” i.e. electronic mail).

Why isn’t there an object‐marker for anwani yako in nimeandika?

Object-markers (OM) are used to replace or emphasize a direct object pronoun when the noun phrase is omitted. When you explicitly state the object as anwani yako, you leave out the OM. OM’s are optional or dropped once the noun appears.

How would I say “She has written your address on the letter”?

Use the subject prefix a- (3rd-person singular) plus -me- for the perfect:
Ameandika anwani yako kwenye barua.