Although Polish conjugations are regular in terms of endings, there are alternations in the verb stems that make some verbs a bit difficult to learn and remember. I’m sure you have noticed that some verbs like iść, brać or jechać, when conjugated, have not just one or two but three different stems. The pattern is quite simple:
(on, ona, ono) idzi-e
(oni, one) id-ą
First person singular and third person plural use one stem, when all other forms use another one.
Unfortunately, it is necessary to memorize all three stems to be able to conjugate those verbs correctly.
The most commonly used irregular verbs:
iść (to go) – (ja) id-ę, (ty) idzi-esz
jechać (to go, to travel) – (ja) jad-ę, (ty) jedzi-esz
jeździć (to go, to travel) – (ja) jeżdż-ę, (ty) jeźdz-isz
brać (to take) – (ja) bior-ę, (ty) bierz-esz
prać (to wash clothes) – (ja) pior-ę, (ty) pierz-esz
móc (to can) – (ja) mog-ę, (ty) moż-esz
nosić (to carry, to wear) – (ja) nosz-ę, (ty) nosi-sz
musieć (to have to, to must) – (ja) musz-ę, (ty) musi-sz
(po)prosić (to ask for, to please) – (ja) prosz-ę, (ty) prosi-sz
If the exercise doesn’t work, you may find it here