The [intensifying interjection] “Oh, verily! (a’-lā)” is for alerting or warning: see how with its warning it declares false all their propaganda, which is alluded to by the word(s) “they say (qālū).”
The [corroborative particle] “innā,” which is for verification: see how it refutes their well-known claims, indicated by “innamā.” It is as though the “innā” is saying that in reality their inner state is one of corruption, so their outwardly being righteous will not profit them.
The restrictive meaning of the personal pronoun “they (hum)” counters the insinuations implicit in “naḥnu” and “innamā:” “We are but improving things.”
The definite article in “(lit.) the spreaders of corruption (al-mufsidīn,” which signifies that you see in them the quintessence of those who cause corruption and that they are in essence depravity – see how this contradicts the restriction expressed by “innamā.”
And “but they perceive it not (wa lākin lā yash‘urūn),” how it contradicts their declaring false the one admonishing them and their claim that they are not in need of advice, since their assertions [about what they are] are known.