Same sentence, different use of conditional


Are both of the following sentences correct?

> a. 明日授業に出たら、予習したほうがいいですよ。
>
> b. 明日授業に出るのなら、予習したほうがいいですよ。

I feel like (a) does not sound good, but I cannot say why.




The second sentence (b) sounds natural, whereas the first (a) doesn’t, at
least not for what you wanted it to mean. It would mean you would do 予習 at the
class, which would be too late if it refers to preparations for that class.

