Corrected sentence:
The guards couldn't believe their eyes when an amazing cat wearing boots and carrying a bag walked by, so they let him inside.
Why this is correct (easy steps):
- Capital letter: Every sentence starts with a big letter, so we keep The at the start.
- Couldn’t is fine: It's the short way to say could not, and it's okay to use in a story.
- Describe the cat clearly: Saying "wearing boots and carrying a bag" tells us what the cat had on and what it held. Putting both actions together keeps the sentence smooth.
- Comma before so: We put a comma before "so" because the sentence has two parts that could be separate. The comma helps them read nicely.
- Word for the cat: We used "him" because the story probably means a male cat. If you don’t know the cat’s gender, you can use "it" instead.
Another simple way to say it:
The guards couldn't believe their eyes. An amazing cat in boots carrying a bag walked by, and they let him inside.
If you want, I can make the sentence shorter or more fun for a story aimed at kids your age.