Explanation

• The boy had watched the match before his friend came
• – لڑکا اپنے دوست کے آنے سے پہلے میچ دیکھ چکا تھا 
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• "Had" is best to keep the meaning of the original sentence
• the above sentence is related to a past sentence so we put "had" 
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• Had is the past tense and past participle of have.
• Had is sometimes used instead of 'if' to begin a clause that refers to a situation that might have happened but did not.