Perfective and imperfective aspect

Two kinds of verbs must be distinguished in Polish: Perfective and imperfective ones. This distinction is usually referred to as aspectual (therefore, you will often hear that a given verb is in the perfective or imperfective aspect). Aspect is a very important linguistic property because it influences both the meaning and the syntactic use of every verb. The simplest explanation of this phenomenon is as follows: A perfective verb describes a single completed action, whereas an imperfective one describes an action in progress (or an action that is habitual). This distinction is illustrated below:

(imperfective)	John czytał książkę.	John was reading a book.
(imperfective)	John zawsze czytał książki, kiedy był dzieckiem.	John always read books when he was a child.
(perfective)	John przeczytał książkę.	John has read a book.

Both czytać and przeczytać mean to read. However, przeczytać is perfective, which means that it can only designate a completed action. This is why perfective verbs are never used in the present tense. You can refer to present actions/events/states only with imperfective verbs.

John czyta książkę.
John is reading a book.

John zawsze czyta książki.
John always reads books.

Most verbs that have been introduced in previous lessons are imperfective (since most sentences that have so far appeared in this course are in the present tense). Perfective verbs are very often derived from imperfective ones by adding various prefixes:

	Imperfective	Perfective
-prze-	czytać	przeczytać	to read
-wy-	pić	wypić	to drink
-na-	uczyć się	nauczyć się	to learn
-o-	budzić się	obudzić się	to wake up

However, it does not need to be the case, as the following pair shows:

	Imperfective	Perfective
	zaczynać	zacząć	to start

The sentences below illustrate the semantic difference between perfective and imperfective verbs:

John pił piwo, ale go nie wypił.
John was drinking a beer, but he didn’t finish it.

John uczył się języka angielskiego, ale nigdy się nie nauczył.
John was studying English, but he never learned it.

John zawsze zaczynał pracę o ósmej rano, ale raz zaczął o siódmej.
John always started work at eight in the morning, but once he started at seven.

As you can see, you use an imperfective verb to indicate that a given action was in progress at some point (or that it was habitual), while a perfective verb is used when you want to indicate whether the action in question was completed.