Unless, how to use?

I've got two sentences:

"Unless it is sunny, we will not go out"
"Unless it is not rainy, we will not go out"

My local teacher of English told me that the second one was wrong. You can't use a negative clause after 'unless'. Is it true? I am confused 'cause I've always been thinking that the structure: 'unless + not' is OK. Can you help me out?

Any comments? Unless you prefer not to say anything.

Can I take it for an answer?

We need some help!

I don't know if the 2nd one is actually wrong, but it's too complicated -- too many negatives. It almost sounds like a linguistic puzzle or trick sentence.

You might say "If it's rainy, we won't go out" or "If it's not rainy, we'll go out", instead.

I'd tend to say that I'd usually say that statement like this:

We will go out, unless it's rainy.

any thoughts?

unless = if not

Unless is similar in meaning to if not and can be used instead of if not in certain types of conditional sentences. We normally use unless with present tenses when we are referring to the future:

You won't get in to see the show, if you don't have reserved seats. OR:
Unless you have reserved seats, you won't get in to see the show.

Let's play tennis on Saturday, if it's not raining. OR:
Let's play tennis on Saturday, unless it's raining.

I'll see you at the gym this evening, if you're not too tired. OR:
I'll see you at the gym this evening, unless you're too tired.


if not

However, we CANNOT use unless in questions:

What will you do if you don't pass those exams?

If I don't pass those exams, I won't be able to study in Australia

I won't be able to study in Australia, unless I pass those exams.
And we cannot use unless with would to talk about unreal future situations:

If he didn't take everything so seriously, he would be much easier to work with.

If he weren't so bad-tempered, I would help him to get the work done
We cannot use unless with would have to talk about unreal situations in the past either:

If you hadn't driven so recklessly, you wouldn't have had this accident.

If you hadn't had that last glass of wine, this would never have happened.

thank you very much, Adam. Your reply helps me a lot.