"His tie"

Translation: Tai yake

[WynneBeers]

Why is it tai yake but koti langu? How do we know which version of the possessive to use?

[juryrigging]

Tai is N/N, so
Tai yake -- his tie
Tai zake -- his ties

Koti is Ji/Ma, giving you
koti langu -- my coat
makoti yangu -- my coats.

Go with it for now. As you progress through the course you'll understand more about noun classes and it will become easier. Alternatively, use other resources in conjunction with Duo to help you follow along better.