"to" is more formal. In conversation people gravitate towars bare infinitive for the sake of efortlessness.
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Mieszko Powroznik
I have just made a google search for this one and found the following reply in one of other forums.
http://www.english-test.net/forum/ftopic36784.html
Mr Wordy was right as to the naturalness of the version with "to".
Anonymous"All you have to do is cook your wife dinner"Because it's optional in the construction known as a "pseudo-cleft with do". You can use to or you can leave it out.
Why is there no 'to' after the be verb?