Short answer: Both sentences are okay, but we can make them a little better. Each sentence should start with a capital letter and usually we say "the winter." Here are corrected versions and why:
- Corrected sentence 1: The bear sleeps through the winter in a cozy den.
- Corrected sentence 2: The bear sleeps all winter in its cozy den.
Why these changes?
- Start with a capital letter: "The" (every sentence starts with a capital).
- We usually say "the winter" instead of just "winter." Both meanings are the same, but "the winter" sounds more natural.
- "Through the winter" and "all winter" mean the same thing — the bear sleeps the whole winter season.
- Using "its cozy den" or "a cozy den" gives a little more detail. "Its" tells us it is the bear's den.
Pick one of the corrected sentences to use. Both are clear and grammatically correct for a simple sentence about a bear.