〜のは〜だ nowa ... da
〜のは〜だ（nowa ... da）means “the one that ...; what ... is (nominalizing + focus)”. の turns a clause into a noun, and は marks it as the topic so that the part after it is highlighted. Formation: ［普通形（ナAな／Nな）＋のは］＋［説明・理由・強調］＋だ／です
the one that ...; what ... is (nominalizing + focus)
N4Formation
［普通形（ナAな／Nな）＋のは］＋［説明・理由・強調］＋だ／です
Explanation
の turns a clause into a noun, and は marks it as the topic so that the part after it is highlighted. Use it to single out and emphasize what, who, where, or why. Na-adjectives and nouns before のは take な (好きなのは, 学生なのは).
Examples
Note
Before のは, na-adjectives and nouns take な; verbs and i-adjectives use the plain form directly.
FAQ
〜のは〜だ — what does it mean and how is it used?
the one that ...; what ... is (nominalizing + focus). の turns a clause into a noun, and は marks it as the topic so that the part after it is highlighted. Use it to single out and emphasize what, who, where, or why. Na-adjectives and nouns before のは take な (好きなのは, 学生なのは).
〜のは〜だ — how do you form it?
［普通形（ナAな／Nな）＋のは］＋［説明・理由・強調］＋だ／です
〜のは〜だ — any usage notes?
Before のは, na-adjectives and nouns take な; verbs and i-adjectives use the plain form directly.