请教一道英语同义句转换题和英语知识easy~```

请教一道英语同义句转换题和英语知识easy~```
To keep our environment clean and tidy is our duty.(改为同义句)[每空一词]
_____ ______ our ______ ______ keep our environment clean and tidy.
另问一道："Do you have an eraser?"he asked me.(合并成一句)
He asked me _____ ____ _____an eraser.
当"if"作“是否”讲时,其后应接何时态呢?