Possum, posse, potui (able, capable) is a common verb found in Latin text. Often, it is the first irregular verb encountered by the elementary Latin student and can provide some frustration on its function and proper grammatical use in a Latin sentence. However, the irregularity of the verb is quite simple and worth memorizing as the verb will come up repeatedly in most Latin study programs.
Possum is a compound word from two Latin words: potis meaning capable and sum meaning to be. The irregularity of the verb gives students trouble because the verb does not follow a standard conjugation like other verbs. For forms of the word sum that begin with s, the t is changed to another s. This change is called assimilation. Recall that the verb sum is conjugated as:
If the verb were regular, the 1st-person singular would be potsum but notice that since sum starts with an s, the t becomes an s as in possum. The rest of the singular conjugation remains regular as in potes and potest. In the plural, what would be potsumus is changed to possumus, potestis is regular, and potsunt becomes possunt. The full conjugation is given below:
Possum is often coupled with an infinitive verb to indicate what is able or capable. The infinitive, when coupled with possum, is called a complementary infinitive. Think of the complementary infinitive as completing the phrase. For example:
Femina bona virum malum superare potest
The good woman is able to conquer the evil man
Notice that superare (conquer) indicates what the subject of the sentence (femina) is able to do. Also, note that the infinitive does not have a subject of its own. The subject of the complementary infinitive is the same as that of the verb possum.
Luckily, the formations of the future and imperfect tenses are regular with regard to possum. The future is conjugated as any other future verb:
as is the imperfect:
The present conjugation of possum is worth memorizing as it is a common construction when coupled with a complementary infinitive. Like in English, the construction is so common that it is often taught early in Latin studies so its form can be easily recognized in a Latin text.