An Auxiliary verb. Like be and have, do can be used both as an auxiliary and as a main verb. It is a common and useful main verb in sentences such as: Overcoats will do more than keep you warm this winter.
As an Auxiliary verb it is used:
 To make negative statements:
Water lilies do not grow well if water is falling on to their leaves.
 To form questions:
‘Do you understand me?’ he asked a second time.
 To form Tag questions:
Well, it doesn’t matter about anyone else, does it?
 For emphasis:
And he does like to travel.
 To avoid repetition:
I think you all know him better than I do.
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