When to Use Auxiliary Verbs: Sank or Sunk? Sang or Sung? Drank or Drunk?

The boat has sunk.
The boat sank.

He has drunk too much.
He drank too much.

She has sung that song many times before.
She sang very well.

All of these sentence pairs are grammatically correct, but switch the words in italics and the sentences are no longer correct. Sunk, drunk, and sung are all examples of past participles which require an auxiliary verb (such as has, had, was, etc.)