Every verb has two stems on the base of which its inflectional forms are made. These are the infinitive stem and the present stem. Both of them have to be remembered when learning a verb and are usually shown in the dictionary entries.
Individual forms are made always using only one of the stems, for instance the past compound uses only the infinitive stem, whereas the imperative uses only the present stem.
Present stems are identical to the 3 sing. pres. forms and are divided into four classes according to their last vowel:
I -e- stems, ex.: njese carries, present stem: njese-;
II -i-/-y- stems (-y- only after s and z), ex.: spi sleeps, present stem: spi-;
III -a- stems, ex.: čita reads, present stem: čita-;
IV irregular verbs: je is, da gives, ma has, dźe goes (on foot), jě eats, wě knows, chce wants, smě may.
Most of the verbs have the infinitive form ending in -ć (ex.: widźeć to see, spać to sleep). The infinitive stem of those verbs is the part of the infinitive form that remains after cutting the final -ć (eg.: widźe-, spa-).
Some verbs have the infinitive form ending in -c (eg. pjec to bake, móc can, to be able to). The infinitive stem of these verbs is made by replacing the final -c with -k- or -h- (ex.: pjek-, móh-).
stems ending with a consonant, ex.: njes- (njesć) to carry, přas- (přasć) to spin, pjek- (pjec) to bake, móh- (móc) can, to be able to. When the last consonant of an infinitive stem is -s- it may in other forms that are made out of the stem:
a) not to change, ex. njesć – je njesł to carry – has carried, as the present stem has
b) change into -t- or -d-, ex. přasć – je přadł to spin – has spun, as the present stem has -d- here: přadu I spin (the -t- and -d- can furtherly alternate with -ć- and -dź- respectively: předźe spins).
stems ending with one of the following vowels: -i- -y- -ě- -e- -a-, ex.: wuči- (wučić) to teach, nosy- (nosyć) to carry, widźe- (widźeć) to see, kćě- (kćěć) to bloom, woła- (wołać) to call.
A dictionary entry of the verb wzać would look like this:
„wzać wozmje perf. to take”
which means that the infinitive stem of this verb is wza- and the present stem is wozmje-.