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I hope it doesn't rain tomorrow.
Literal
Tomorrow rain [subject-が] fall-not-if good [quotative-と] think.
降らなければいいと思う — 'I think it would be good if it doesn't rain.' Uses the ~なければ conditional of the negative form. Compared with the simpler ~といいな wish-form, wrapping the wish inside ~と思う adds a measured, opinion-stating tone — less wistful, more deliberative.