Blog entry by Josef Milon | Language Hub
There are often questions about the long and short vowels in the Czech language. It is uneasy to give students the patterns and rules to follow, which come out of a long row of forms and models.
Good news is that in verbal infinitives, we are able to define one scheme:
there is almost always a long vowel in one-syllable infinitive and its prefix variants (hrát, vyhrát, pít, vypít).
Note, that two- or more- syllable verbs usually have short last vowel: pracovat, udělat, telefonovat.
Exceptions are as follows: smět, pět (archaic version of pít), psát x napsat.
Anyway, it is always better to check the correct version on http://prirucka.ujc.cas.cz/.