The correct answers are I watched the suspect sneak into the bakery and steal a cupcake. and I watched the suspect sneaking into the bakery through the back door.

The verb watch is a verb of perception. The grammatical pattern is: watch + object + bare infinitive OR watch + object + -ing form.

• "Sneak" (bare infinitive) focuses on the completed action.

• "Sneaking" (-ing form) focuses on the action in progress.

You cannot use a to-infinitive ("to sneak") or a conjugated verb ("sneaks") after the object.