。
I don't think it'll rain tomorrow.
Literal
Tomorrow rain [topic-は] fall-not [quotative-と] think [assertion-よ].
雨は降らないと思うよ — the は on 雨 is contrastive ('as for rain specifically, I don't think so'). The よ adds casual assertion. Japanese typically negates the content verb rather than the thinking verb, unlike English 'I don't think it'll rain' — Japanese says '降らないと思う' (think-that-not-rain), keeping the negation inside the quoted clause.