Learn Hardcore Hausa: Yau Musa yana cikin bakin ciki sosai.

Yau – today
Musa – the name Musa
yana – he is / is (doing/being) (3rd person masculine, continuous/progressive aspect)
cikin – in / inside
bakin ciki – literally something like “inner blackness”, idiomatically sadness / grief
sosai – very / a lot / greatly

So a very literal version would be: “Today Musa is in (a state of) deep sadness very much.”
Natural English: “Today Musa is very sad.”

What exactly is yana doing in this sentence? Is it just “is”?

Yana is the 3rd person singular masculine continuous/progressive marker, roughly “he is (doing/being)”.

In this sentence:

Musa is the subject.
yana marks present, ongoing state.
The “thing he is in” is cikin bakin ciki (“in sadness”).

So Musa yana cikin bakin ciki is like saying “Musa is (currently) in sadness”, i.e. “Musa is sad (now)”. It’s more than a simple copula “is”; it carries aspect (ongoing state).

Why is cikin used? Why not just ?