It is the first grammar of an East Slavic language (and not of Church Slavonic, which Uzhevych calls lingua sacra, “sacred tongue”).
Thus, Ivan Bilodid (1972), Mykhaylo Zhovtobryukh (1976), and Vasyl Nimchuk (1985) have stressed Ukrainian features of his language, whereas Aleksandr Sobolevskiy (1906), Vatroslav Jagić (1907), James Dingley (1972) and Yuriy Shevelov (1979) have pointed out its Belarusian aspects.
Thus, on the one hand there are long lists of constructed verb forms that are of no practical relevance for Ruthenian, e.g. the optative pluperfect бодай бымъ былъ ковáлъ (bodaj bym” byl” koval”) ‘oh would I have had cooked!’ (Arras, 452), but on the other hand the locative case is not reflected, and Uzhevych tries to explain the respective endings, which sometimes resemble the dative, sometimes the instrumental (ablative) endings, as casus vagabundi, “wandering cases” (Arras, 332-341).
As Helmut Keipert (2001) has shown, the anonymous and undated manuscript titled Rozmova and Besěda (which linguists so far wrongly attributed to the end of the 16th century) is also an autograph by Ivan Uzhevych.
The manuscript is a parallel translation of the popular phrasebook by Noël de Berlaimont into Ruthenian and Church Slavonic.
Anyway, this phrasebook provides a lot of linguistic material to fill the gaps in the Grammatica sclavonica, which itself contains only little actual Ruthenian text, as it is written in Latin.