There aren't any specific words that you *can't* begin a sentence with. Any word can technically be the first word of a sentence, depending on the context.
However, certain words are generally considered less formal or grammatically "incorrect" to start a sentence with, like:
* Conjunctions: And, but, or, so, yet, for
* Prepositions: On, at, in, with, to
* Pronouns: He, she, it, they, we, you
It's important to remember that these are just guidelines, and there are always exceptions. Sometimes these "unconventional" sentence beginnings can add emphasis or create a certain effect.
Let me know if you have any other word puzzles for me!