В носу́ - дательный, в голове́ - дательный или предложный? Is there a rule for this usage?
В носу, в голове is the prepositional case. In the old days it was a separate case, the locative case (местный падеж). Then it was combined with the prepositional case, because most of the forms were the same (о голове - в голове). But not all of them were the same! These forms in modern Russian are a special group of the prepositional case: в лесу, в снегу, в раю, в носу, на двери, на краю.
Their differences are as follows:
- The ending -у/-ю (-и for feminine nouns like дверь).
- The preposition в/на; by this feature they can be distinguished from the dative - the dative is never combined with these prepositions.
- The accent is always on the end; by this feature they also can be distinguished from the dative: к лЕсу (dat.) - в лесУ (prep. loc.).
- Their meaning is different from that of the standard prepositional form: в мИре (prep.) =/= в мирУ (prep. loc.), на крАе (prep.) =/= на краЮ (prep. loc.).