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She tried to lift the box but found it impossible.
Literal
She [topic-は] that box [object-を] lift-up [tried-to-ようとした] [but-が] impossible [quotative-だと] realized.
持ち上げる ('lift up') is a compound verb: 持つ ('hold') + 上げる ('raise'). ~ようとする ('try to') marks an attempted action — past form ~ようとした puts the attempt in the past. Clause-final が ('but') links the thwarted attempt to the discovery that followed. ~とわかる ('realize, find out') uses the quotative と to embed the conclusion: 'realized that it was impossible.' Note that 不可能 ('impossible') is a stronger word than 無理 ('not possible, unreasonable'), with a more absolute, almost technical flavor.