"Look behind you."
Translation:अपने पीछे देखो।
By looking at the form of the verb. Hear देखो is used, which corresponds to तुम. For तू it would be देख so just the stem of the verb. Finally, for आप one would use देखिये. Here you can read about all those forms:
According to the link above, there is also so called deferential imperative but it's rarely used.