Almost every Upper Sorbian verb is inherently eather imperfective (impf.) or perfective (pf.) (there are few verbs that are ambiguous in this respect, ie. may be both impf. and pf.). The verbs' quality of being perfective or nonperfective is called verb aspect and is alaways shown in dictionary entries.
Perfective and imperfective verbs reffering to the same kind of action are not two different forms of the same word. They are two different words. For instance, both impf. brać and pf. wzać translate to English as to take but it is clearly visible they are not the same verb. However, most of the impf.–pf. verb pairs share the same root modified with different additional morphemes, ex. impf. pisać vs. pf. napisać to write, impf. zapaleć vs. pf. zapalić to set on fire.
A perfective verb describes an action, that doesn't recure and:
has just finished or started;
finished or started some time ago;
will have finished or started;
will finish or start.
As we can see, a perfective verb apllies only to the past or future and not to the present.
A nonperfective verb describes an action that recures, recured or will recure or lasts for some period in the past, present or future.
Compare the use of pf. and npf. verbs in the following Upper Sorbian translations of English sentences:
[nan father; kniha book; brać bjerje npf. to take; wzać wozmje pf. to take]
A PAST ACTION:
The father took a book.
Nan wza knihu.
The father has taken a book.
Nan je wzał knihu.
The father was taking a book.
Nan bjerješe knihu.
The father has been taking a book.
Nan je brał knihu.
The father used to take a book (on every Sunday).
Nan bjerješe knihu (kóždu njedźelu).
A PRESENT ACTION:
The father takes a book (on every Sunday).
Nan bjerje knihu (kóždu njedźelu).
The father is taking a book.
Nan bjerje knihu.
A FUTURE ACTION:
The father will have taken a book.
Nan wozmje knihu.
The father will take a book (on this Sunday).
Nan wozmje knihu (tu njedźelu).
The father will take a book (on every Sunday).
Nan budźe brał knihu (kóždu njedźelu)
The father will be taking a book.
Nan budźe brał knihu.