translation " key of the Janus" [classical Latin]

1. Clavis Ianus
2. Clavis Iani
3. Clavis Ianvs
4. Clavem Ianus

If you want to say "key of Janus", you need the nominative (subjective)
case of clavis and the genitive (possessive) case of Ianus.

1. both are nominative
2. correct
3. uses an alternate way of writing Ianus, but it is still nominative case.
4. uses the accusative (objective) case of clavis and the nominative case
of Ianus.

so correct translation is

"Clavis Iani"

or if I understand you correctly the declensions of "Clavem Ianus" must be conversely. nominative for clavis and the accusative (objective)
for Ianus. but how, I not really sure...

am I right?

Ianus - nominative
Iani - genitive
Iano - dative
Ianum - accusative
Iano - ablative

thank you!

You're welcome, viktor80.