The past tense of grin like a Cheshire Cat is grinned like a Cheshire Cat.
The third-person singular simple present indicative form of grin like a Cheshire Cat is grins like a Cheshire Cat.
The present participle of grin like a Cheshire Cat is grinning like a Cheshire Cat.
The past participle of grin like a Cheshire Cat is grinned like a Cheshire Cat.