Complex sentence: By the time of Rabindranath Tagore, the shadhu (pure) part of Shadhu bhasha had largely waned into just a set of verb forms and in a decade or two he himself would choose Cholito bhasha as a literary form of Bengali.
Split into simpler sentences: By the time of Rabindranath Tagore the shadhu (pure) part of Shadhu bhasha has waned into just a set of verb forms. And in a decade or two he himself would choose Cholito bhasha as a literary form of Bengali.