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In addition, you should be sure that if you have a subject and a past participle, you also have the verb have. This problem is particularly common with those verbs (such as sing sang, sung) that change from present to past to past participle by changing only the vowel.

My friend sung* in the choir. => (should be sang or has sung)
He become* angry at his friend. => (should be became or has become)
The boat sunk* in the ocean. => (should be sank or has sunk)