Each verb is either perfective or imperfective, and most verbs occur in pairs to express the same meaning with different aspects. For example, the concept of "jumping" is expressed in the 2 different aspects is "skákati", which has an imperfective aspect and can roughly be translated as "to be jumping (continuously)", and "skočíti", which has a perfective aspect and can roughly be translated as "to jump (once)". While each aspect is represented by a full verb with its own distinct conjugation, certain combinations are not or rarely used in one aspect or the other. For example, imperfective verbs generally lack a past passive participle, while perfective verbs have no present participles. Additionally, the present tense has 2 different meanings depending on the aspect of a verb. For imperfective verbs, it has present meaning, while for perfective verbs, it has a future meaning expressing a desire to carry out the action. For example, "To kravo prodam" "I want to sell the cow" (compare this with the future tense "To kravo bom prodal" "I will sell the cow").