ANSWER: You can recite it throughout Ramadan and even during the other months but should say it more during the last 10 nights of Ramadan.

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ANSWER: The last ten nights of Ramadan begin from night of 20th fast (in hijri calendar the new day begins at maghrib)

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ANSWER: He doesn’t have to add anything as he prayed one single rakah with his imam and that was sufficient. If the imam prayed 3 rakah and he only caught one, he must stand up after salam of the imam and compete the remaining two. 

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