The past tense of use a sledgehammer to kill a gnat is used a sledgehammer to kill a gnat.

The third-person singular simple present indicative form of use a sledgehammer to kill a gnat is uses a sledgehammer to kill a gnat.

The present participle of use a sledgehammer to kill a gnat is using a sledgehammer to kill a gnat.

The past participle of use a sledgehammer to kill a gnat is used a sledgehammer to kill a gnat.