Learn Hardcore Serbian: Roštilj lepo miriše.

Roštilj can mean barbecue, grill, or barbecued/grilled meat, depending on context. In this sentence, it most naturally refers to the barbecue/grilled food giving off a nice smell.

What part of speech is each word here?

Roštilj = a noun
lepo = an adverb
miriše = a verb

So the structure is basically:

noun + adverb + verb

Why is it lepo and not lep?

Because lepo modifies the verb miriše. In English, we also usually use an adverb with a verb: smells nicely/pleasantly—although English normally says smells nice.

lep = beautiful / nice as an adjective
lepo = nicely / pleasantly as an adverb

Serbian commonly uses the adverb here: lepo miriše = smells nice / smells pleasant.

What does miriše come from?

is the form of the verb .