Some masculine nouns end in -ao or -eo in the nominative singular. Historically, this final o was an l. When we decline these nouns—adding endings for other cases or plurals—the o reverts back to l.
For nouns ending in -ao, two changes happen at once to form the new stem: the a drops out (it's a "fleeting a"), and the final o changes back to l. For nouns ending in -eo, the e typically stays while the o reverts to l.
| Nominative | Stem | Genitive | English |
|---|---|---|---|
| kotao | kotl- | kotla | cauldron |
| ugao | ugl- | ugla | corner |
| čavao | čavl- | čavla | nail |
| posao | posl- | posla | work / job |
| pepeo | pepel- | pepela | ash |
| anđeo | anđel- | anđela | angel |
When the word is the subject (nominative singular), it retains the -ao or -eo. In other cases, the stem uses the l.
| The largest cauldron is in the corner. | Najveći kotao je u uglu. |
| There is no ash under the cauldron. | Ispod kotla nema pepela. |
This rule applies to plural forms as well (e.g., adding the accusative plural ending -e to the stem čavl- gives čavle).
| The blacksmith put the nails next to the anvil. | Kovač je stavio čavle pokraj nakovnja. |
Nouns like anđeo and pepeo keep their e but still revert the o to an l before taking case endings.
| The blacksmith forges an angel out of steel. | Kovač kuje anđela od čelika. |
| He sees nothing because of the ash. | Ne vidi ništa od pepela. |
Notice how both posao and ugao undergo the exact same transformation in the following sentence.
| He did a lot of work in that corner. | Napravio je puno posla u tom uglu. |