CC Ādi 16.56

‘gaṅgāra mahattva’ — śloke mūla ‘vidheya’
idaṁ śabde ‘anuvāda’ — pāche avidheya
Word for word: 
gaṅgāra mahattva — glorification of mother Ganges; śloke — in the verse; mūla — chief; vidheya — unknown; idam śabde — by the word idam (“this”); anuvāda — the known; pāche — at the end; avidheya — improper.
Translation: 
“The glorification of the Ganges [mahattvaṁ gaṅgāyāḥ] is the principal unknown subject matter in this verse, and the known subject matter is indicated by the word ‘idam,’ which has been placed after the unknown.