For the purposes of the Database, defective verbs are verbs for which we cannot determine a theme vowel class.

These verbs are marked as defective in the Theme Vowel column.

For BCS, these are the defective verbs: sam, nisam, jesam, biti, hteti/htjeti.

For Slovenian, these are the defective verbs: biti 'to be', imeti 'to have' and iti 'to go'.