This is the 417th most frequent Czech word.
hlavním
"Main" or "chief."
Here, 'hlavním' is used as an adjective meaning 'main,' modifying 'městem,' which is in the instrumental case.
Hlavním městem České republiky je Praha.
The capital city of the Czech Republic is Prague.
In this sentence, 'hlavním' functions as an adjective in the instrumental case, modifying 'tématu,' indicating the primary topic of the discussion.
Diskutovali o hlavním tématu schůze.
They discussed the main topic of the meeting.
Here, 'hlavním' again serves as an adjective meaning 'main,' modifying 'úkolem,' which is in instrumental case due to its role as the complement of the subject linked by the verb 'je' (is).