| Sg. Indef. | frumskógur |
| Sg. Def. | frumskógurinn |
| Pl. Indef. | frumskógar |
| Pl. Def. | frumskógarnir |
| Sg. Indef. | frumskógar |
| Sg. Def. | frumskógarins |
| Pl. Indef. | frumskóga |
| Pl. Def. | frumskóganna |
| Sg. Indef. | frumskógi |
| Sg. Def. | frumskóginum |
| Pl. Indef. | frumskógum |
| Pl. Def. | frumskógunum |
| Sg. Indef. | frumskóg |
| Sg. Def. | frumskóginn |
| Pl. Indef. | frumskóga |
| Pl. Def. | frumskógana |