ANSWER: Because tahiyatul masjid is mandatory while the sunnah prayers are voluntary! The prophet salla Allahu alaihi wa sallam prohibited you from sitting down before offering it. 

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ANSWER: If the front row is full then you must pray alone in the second row and not pull anyone back from the first row as this is prohibited.

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ANSWER: You recite the same that you recite in the sujood of prayers and rabbigh firli in between both prostration.

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