Learn Finnish: Hän on muka nähnyt ufon, mutta se lienee hölynpölyä. - He supposedly saw a UFO, but it is probably nonsense.

The particle muka expresses doubt, disbelief, or indicates that something is merely a claim. While Hän on nähnyt ufon is a factual statement ('He has seen a UFO'), adding muka changes it to 'He claims to have seen a UFO' or 'He supposedly saw a UFO.' It clearly signals that the speaker doesn't believe the claim.

What verb form is lienee and how does it fit with the rest of the sentence?

The word lienee is the third-person singular form of the verb olla in the potential mood, which expresses probability ('is probably' or 'might be'). It perfectly complements the doubt introduced by muka in the first clause, shifting the sentence from doubting the claim to stating what the speaker thinks is actually true.

Why is ufo written as ufon with an -n ending instead of the partitive case?

It is the total object of the sentence. The action of seeing the UFO is viewed as a complete, finished action in the perfect tense (on nähnyt). Because the action is complete and the object is a whole, countable thing, it takes the accusative case, which looks identical to the genitive -n for singular nouns.

Why is hölynpölyä in the partitive case?

It acts as a predicative describing the subject se ('it'). Because hölynpöly ('nonsense') is an abstract, uncountable concept rather than a discrete object, its predicative form must be in the partitive case.