'Do' is the root form. It is also the form used with 'I', 'you', 'we' and 'they'. 'Does' is used only with singular third person pronouns, ie. 'he', 'she' and 'it'. 'Did' is the past tense of 'do', used to refer to an action in the past. eg. 'I do this today. I did this yesterday.' Hope this helps! :) P.S.: in your question, you should use the word 'difference' instead of 'different'.