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The taller the tree, the harder it is to climb.
Literal
Tree [topic-は] tall-[if-ば] tall-[to-the-extent-of-ほど] climb-hard.
~ば~ほど is the standard 'the more X, the more Y' frame in Japanese. You conjugate the adjective or verb into its ば-form, repeat the dictionary form, and attach ほど — here 高ければ高いほど ('the higher the higher') — and then give the resulting claim. ~にくい ('hard to X') is the opposite of ~やすい.