Can you cast a modal double-faced card from exile if only one face says you may cast it?

Modal double-faced cards (MDFCs) have two independent faces, and each face is evaluated separately for purposes of casting permissions granted by exile effects. When a spell or ability says 'you may cast this card from exile,' it authorizes casting, but only the faces that the permission applies to can be cast. CR 712.4 establishes that each face of a modal double-faced card is treated as its own object for most purposes, including what you are allowed to cast.

If an effect exiles an MDFC and says 'you may cast the top face from exile' or simply 'you may cast it' while referring to the face with the permission printed on it, only that face may be cast. The other face has no independent authorization from that effect. CR 601.3a clarifies that to cast an MDFC from a zone other than hand, you must be explicitly permitted to cast it from that zone, and that permission is face-specific unless the granting effect says otherwise.

Conversely, if an effect grants broad permission — such as 'you may cast this card from exile' without specifying a face — you may choose either face when casting the MDFC, because the permission attaches to the card as a whole and CR 712.2b allows you to choose which face to cast at the time you begin casting.

Example: Suppose Jwari Disruption // Jwari Ruins is exiled and only the instant face (Jwari Disruption) carries a 'you may cast this face from exile' clause. You may cast it as the instant, but you cannot flip it around and play the land face or cast it as the other face — that face has no casting permission from exile.

In summary: the permission is scoped to the face it appears on or the face the granting effect references. Only that face is castable from exile under that specific permission (CR 712.4, 601.3a).