Up to now, Czech linguistics generally assumed that the so called secondary imperfective verbs (SI) can only be used if the primary imperfective verb (PI) and the SI of the same root have different meaning. Thus, the SI svařovat (S-vař-ova-t (to boil; -vař- being the root, s- the prefix and -ova- and -t the suffixes) is used because the PI vařit (vař-i-t (to cook) has a different meaning. Contrary to that, uvařovat (*U-vař-ova-t) is not used because its meaning would be the same as in the basic PI vařit. Only a few exceptions to this rule were assumed to exist. However, the corpus findings show that this assumption was false. There are at least 160 SI derived from the basic PI in current standard written Czech language which are synonymous to the PI. This article concentrates on SI derivations with the prefix VY- and presents empirical data which enable us to postulate the synonymy of PI and SI with some 70 verb pairs.