لن يفسل المجتهد" - ما أتر "لن" على الفعل؟"

In Arabic grammar (علم النحو), the particle "لن" is one of the particles that govern the imperfect verb (الفعل المضارع). Specifically, لن is a حرف نصب (accusative particle) - it negates the future and puts the imperfect verb into the accusative case (المنصوب).

In the sentence "لن يفشل المجتهد" (the diligent person will not fail), the verb يفشل is in the منصوب (accusative) case because of لن. The sign of نصب on a regular imperfect verb is the فتحة on the final letter: يفشلَ.

The effect of لن is therefore that it puts the verb in the accusative (تنصبه). It does not raise it to the nominative (ترفعه) - that is the default state of المضارع when no governing particle precedes it. It does not put it in the jussive (تجزمه) - that is the function of particles like لم and لا الناهية. And it does not delete it (تحذفه) - no grammatical particle deletes a verb entirely.