"Plover": o/a/s

This root alternation dates back to a time when the common ancestor of Slavic languages had a more developed system of vocalisms: there was a developed system of diptongs and the division of vocals into dogian and short, and the system of accents was fundamentally different from the modern (the place of emphasis could perform a significance distinctive function). Over time, in many words, differences in the place of emphasis led to differences in vocal, especially in A and O - under emphasis they began to be pronounced as long, without emphasis - as short.
At the next stage of development, with the disappearance of long and short, there was, at first glance, an unexpected transformation: the long A and O reflected into the modern A, the short A and O gave birth to the modern O. The traces of this process we observe in many Russian roots.

However, the alternation of float/float/float was complicated by two factors: the flexibility of emphasis (the alternation of float/float in modern language is not associated with the impact of the vocal) and the additional influence of the form of “float” which was mixed with the suffixal forms on the “vati” attached verbs of an imperfect kind (breaking – breaking), although in float/float/float B – the root, not the suffixal. Under the influence of this mixture and appeared "Y" in the root swim, swim.