Can a dentist prevent a tooth that feels like it could crack anytime if bittenn down too hard?

Usually: While your question is somewhat unclear, it sounds like you are asking the question, "can a dentist prevent a fragile tooth from cracking?" the answer is yes, if there is enough sturdy tooth structure present. There are several methods, the best one usually being a crown. You need to ask your own dentist regarding that specific tooth, as we can only guess as to the condition of your tooth.

Maybe: Cracked teeth are difficult to diagnose, and even harder to treat. The most common course of treatment is a full coverage restoration, usually a crown. If the tooth starts to hurt, a root canal treatment is the next step. Sometimes, despite all of the effort, the crack continues to grow, and the tooth ends up needing to be removed.

Maybe: Need to see the dentist. The tooth may not even be cracked at all; just feels like you are biting too hard on it. Even the slightest sense of biting too hard can be very aggravating. It may just need a slight adjustment. If tooth is indeed cracked, it will need a full coverage crown, and maybe root canal. If a crack extends into the root, the tooth is usually hopeless.