Sometimes you need "did", sometimes just the past tense. Most times, you use the past tense; "did" is for questions, negation, or emphasis. "Did you see the film?" (question) "Yes, I saw it." "Marc did not see the film." (negation) "Yes, he did! I watched it with him!" (emphasis)

"Did" is used in three situations: 1) For emphasis. e.g. A: I saw that film. B: I don't believe you. A: I DID see that film. 2) In questions (http://www.italki.com/question/320504): E.g. "Did you see that film?" 3) In negative sentences: E.g. "I didn't see this film."