If he _______ harder, he would have passed the exam.
If + past perfect, would have + past participle.
She gave me a _______ explanation.
"Explanation" is a noun, an adjective ("clear")+noun
She bought _______ orange from the supermarket.
"Orange" starts with a vowel sound, "an."should correct article with vowelsound.
She worked hard _______ she failed the exam.
"but" is used to contrast two opposing ideas. but is conjunction.
He is proficient _______ Hindi.
The correct phrase is "proficient in" when referring to a subject or skill.
Harry is __________ honest man.
"Honest" starts with a silent 'h' with a vowel sound, so "an" is article used with vowel sound.
He left early __________ he was feeling uneasy.
"Because" is used with an action.
Neither of the boys __________ present in the lab yesterday.
"Neither" is singular, + of +plural noun +singular verb.
The project manager was not satisfied __________ his team's performance.
preposition after "satisfied" is "with"
He __________ the meeting before I arrived.
The past perfect tense ("had completed") is used to show that one action happened before another past action (arriving).