Do the following sentences sound natural? If not, please correct them.

１．Great reversals can happen.
３．I don't believe those words.

See a translation

1. Great reversals can happen.

文脈によれば自然に聞こえる可能性がありますが、一人の文章としては曖昧と不自然に聞こえます。
This could be natural depending on the context, but as a lone sentence it sounds vague and unnatural.

「reversals」では通りたい意味を分かりませんから新しい文章をあげられません。
I can't give you a new sentence because I'm not sure what you mean by "reversals."

３．I don't believe those words.

文脈によれば自然に聞こえる可能性がありますが、一人の文章としては少し不自然に聞こえると思います。
This could sound natural depending on the context, but as a lone sentence I think it sounds a bit unnatural.

もっと自然な文章は「I don't believe what he said.」です。

その他は自然に聞こえます。
The others sound natural.