When to use the past tense and past participle?

You use the past tense of a verb (e.g. sang) when the subject is being talked about in the past (the simple past tense). You use the past participle (e.g. have/has sung) when the subject is being talked about in the present (the present perfect tense). "Sung" is the past participle and "have/has" is an auxiliary verb that implies that the subject is referring to a past action in relation to the current present state.