Use the verb "sang" by itself to indicate simple past action. Examples: "He sang at the party" and "We sang all night long."
Use the verb "sung" plus the helping verb "had" to indicate past perfect tense. Example: "She had sung that song many times before."
Use "sung" plus the helping verb "have" to indicate present perfect tense. Example: "I have sung in a choir before."
Use "sung" plus the phrase "will have" to indicate future perfect tense. Example: "I will have sung my hit song 100 times by next Friday."