"I got a high grade on an exam, not understanding the subject."
Translation:Eu am luat notă mare pe un examen, neînțelegând materia!
Indeed. No Romanian will say ”am luat notă pe un examen”. The correct form is ”la un examen”.
No, actually when its gerunziu then verbs starting with î in negative form remain î.
That is true not only for negative forms, but for any composite constructions involving prefixes (ne-, pre-, contra-, re-, între- etc) where the original non-prefixed word (mostly verbs, but there could be some nouns as well) began in „î”. In this case, ne- + înțelegând (gerund) but also ne- + înțeles (participle), pre- + împărțit (”to divide”, participle) etc.
Interestingly, in the case of words ending in „î” (which to my knowledge are all verbs), the official spelling was changed some 30 years ago to the „â” variant: „a hotărî” - „to decide” --> „hotărât” (participle), „hotărâre” (long infinitive) etc.