Explanations:
उपरोक्त वाक्य में mainly effects के स्थान पर mainly affects का प्रयोग होगा क्योंकि mainly (adverb) verb (affects) को qualify करेगा जबकि effect (Noun) अत: affects का प्रयोग होगा। Correct sentence– The mainly affects the ankles knees and feet, but may also involve the eyes and even the heart.