In fact, the imperative is formed not from the infinitive, but by adding '-en' (or '-n') to the verb stem.
the finite verb followed by 'Sie': Fragen Sie den Mann da. 'Sie': The imperative form is normally the same as that of a question, i.e. German has a different imperative structure for each of its three second-person forms ('Sie', 'ihr', 'du'), as well as first-person plural and third-person commands: 'Long live the king!' 'So be it.' 'May he rot in hell.' Third-person or other indirect commands are also possible, if less common. 'We ought to do it now.' 'We should leave soon.' 'Let's not forget to ask him how to get to his house.' This first-person plural form is usually constructed with the verb 'to let': 'Let us pray.' 'Let's dance!' Sometimes the speaker is proposing something to one or more people and also including him- or herself.
('You stop that right now!' 'You shut up!') or to suggest that the speaker is unable or unwilling to perform the action The speaker may, on the other hand, sometimes use 'you' either to indicate impatience with an addressee of lower status 'Stick it in your ear!' 'Go jump in the lake!' 'Dice and sauté an onion.' Normally, the speaker is addressing someone directly, but the actual pronoun 'you' is omitted: 'Give me the gun!'
The imperative mood is used to express commands.