فَإِذَا فَرَغْتَ فَٱنصَبْ ٧
Sentence structure
فَإِذَا introduces the time/condition. فَرَغْتَ is a perfect with attached subject تَ; فَٱنصَبْ is the imperative response with understood subject أَنْتَ.
Ayah summary
So when you are free, strive in worship.
فَإِذَا — So when
Grammar: فَ connects and إِذَا introduces a future time/condition.
فَرَغْتَ — you have finished
Grammar: A second-person masculine-singular perfect with attached subject تَ.
فَٱنصَبْ — then strive
Grammar: فَ introduces the response; the imperative has understood subject أَنْتَ.