«فَأَمَّا الْيَتِيمَ فَلَا تَقْهَرْ»
As for the orphan, do not oppress him.
Al-yatīma is accusative after ammā, whose response begins with fa; lā in lā taqhar is prohibitive, making the present verb jussive. This is a prohibition, not a declarative report.
Fronting the orphan as the topic gives the directive particular prominence. Topical arrangement and prohibition are distinct observations and should be explained separately.