。
I don't think it'll rain tomorrow.
Literal
Tomorrow [topic-は] rain [subject-が] fall-not [quotative-と] think.
雨が降らないと思う — negation inside the quoted clause. Japanese typically negates the content verb rather than the thinking verb; English 'I don't think it'll rain' becomes 'I think it won't rain' in the Japanese surface form. Casual present (思う rather than 思います).