Stems of a verb
Each verb has two stems: past and present. These stems are used in verb cojugation and word formation.
The past stem always obtains regularly by removing -an from the infinitive:
In Old Persian, verbs had various stems being made from the same root according to certain rules. For example, by adding -a to the root bar (bordan, to take), we get the present stem bara and by converting the root to its weak form br and adding -ta, we get the past stem brta. The relation between the root and the stems has been lost in the transition of Persian to Middle Persian (and consequently, New Persian) and these rules are no longer intelligible. Therefore, regarding conjugation in New Persian, the present stem of verbs follows no global pattern and have to be learned. To facilitate learning, verbs can be classified into groups in which the present stem follows a certain pattern, with no or some exceptions.
NOTE Each verb whether regular or irregular has one and only one present stem for all persons. Therefore, as opposed to languages like French and Spanish, Persian doesn't have irregular conjugation.