Simple past tense and past participle of brighten.(verb)
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Use "brightened" in a sentence
"Then his expression brightened as Balfour pushed the kitchen door open with his elbow and came out holding a silver tray cluttered with mugs and a steaming pot."
"His expression brightened at the new arrival, nodding with enthusiasm of his own."
"Then her expression brightened, and a smile curved her mouth as color flooded back into her cheeks."