«إذا ساء فعل المرء ساءت ظنونه وصدق ما يعتاده من توهم»
When a person’s conduct turns bad, his suspicions become bad, and he believes the fancies to which he is accustomed.
The couplet links a person’s conduct to the quality of his judgments: harmful actions lead him to suspect others and then to trust his own suspicions.
A prose rendering can state: “When someone behaves badly, he increasingly doubts others and comes to mistake habitual suspicions for facts.” This is a reexpression of the relationship, not a word-for-word metrical translation.