Verb てこそ

At a glance

Verb てこそ means "only by doing ~" or "it is precisely by doing ~". It stresses that an action is a necessary condition for a meaningful result or realization.

Verb (て-form) + こそ、[result]

自分じぶんでやってみてこそ、その難むずかしさがわかる。 Only by trying it yourself do you understand how difficult it is.

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How to form it

Put the verb in the て-form and add こそ. Politeness is shown at the end of the sentence, not in てこそ.

With nouns and な-adjectives, the form is であってこそ: 健康けんこうであってこそ "only when you are healthy".