did and verb+ed in past

[Huda2]

hi,
I'm always confused with this issue .

when I talked about a something in past :
ex.: 1- I finished my work
2- I did finish my work

I don't know when I should use (1), and when I should use (2).

thanks

[Violino]

Take a look at this dialog:

Tom: I finished my work yesterday. (just an information)
Kate: Did you really? I don't know why but I don't believe you.
Tom: You don't have to believe me but I did finish it! (the pressure is put on the word "did", Tom assured Kate that he really, for sure finished his work).