Difficult verb pairs in the Polish Language; iść-chodzić, jechać-jeździć

Think of it this way, there are regular imperfective verbs and then there are super-imperfective verbs. iść is regular imperfective and chodzić is super-imperfective. Most of the time the distinction isn't possible or isn't important but when it is.... it is.

There is no similar sub-division of perfective verbs that I'm aware of.

@Dominic and Mafketis, again, many thanks!

Only to reiterate, this has been for me much like explaining Dative and Accusative cases along with transitive/motion vs. intransitive/rest to my students of German:-)

It takes while NOT to confuse concepts.

that's why perfective verbs don't have present tense forms only past and future

....it's also why certain verbs have either ONLY one form, perfective, OR imperfective, and NOT both!! Most verbs though are paired.
:-)