In almost[clarification needed] all modern Slavic languages, only one type of aspectual opposition governs verbs, verb phrases and verb-related structures, manifesting in two grammatical aspects: perfective and imperfective (in contrast with English verb grammar, which conveys several aspectual oppositions: perfect vs. neutral; progressive vs. nonprogressive; and in the past tense, habitual ("used to ...") vs. neutral).
The aspectual distinctions exist on the lexical level - speakers have no universal method of forming a perfective verb from a given imperfective one (or conversely).
However, each Slavic language contains a number of bi-aspectual verbs, which may be used as both imperfective and perfective.
Aspect in Slavic languages is a superior category in relation to tense or mood.
For example, (omitting, for simplicity, feminine forms like jela): Ja sam pojeo signals that the action was completed.
Ja sam jeo signals that the action took place (at a specified moment, or in the course of one's life, or every day, etc.
When such a verb is supplemented with an explicit aim or direction, an iterative sense is conveyed: chodzić do szkoły 'to go to school (usually, repeatedly, on several occasions)'.
States and actions that are seen as constituting a lexeme-specific block of time can be expressed by means of both imperfective and perfective verbs: cały dzień leżał w łóżku 'he was in bed all day long' (literally: 'he lay in bed') means nearly the same as cały dzień przeleżał w łóżku.
The difference is mainly stylistic: imperfective is neutral here, while using perfective causes stronger tone of the statement.
In most Slavic languages, including Polish, a present perfective verb form may stand by itself as future tense.
In the context of specific verbs, the question of whether any given prefix carries a semantically neutral or canonical perfective sense is not straightforward; distinctions in meaning and dialectical differences influence the choice.
For example, in Polish: Some verbs form their aspectual counterparts by simultaneous prefixation and suffixation, ex.