〜のは〜だ
the one who / the thing that … is …
Plain form + のは + focus + だ / です
Sign up free to review this point on a spaced schedule.
のは〜だ moves the important part of a sentence to the end, like "it was … that" in English: 私が生まれたのは大阪です, "it was Osaka where I was born".
The の turns the first part into a noun ("the one where I was born"), は makes it the topic, and the focus goes before です.
It's how Japanese stresses one piece of information: 昨日来たのは田中さんです, "it was Tanaka who came yesterday" (not someone else).
With から, it gives a reason as the focus: 遅れたのは電車が止まったからです, "the reason I was late is that the train stopped".
Adjectives work the same way: 日本語で難しいのは漢字です.
Examples
私が生まれたのは大阪です。
It was Osaka where I was born.
一番好きなのは夏です。
The one I like best is summer.
昨日来たのは田中さんです。
It was Tanaka who came yesterday.
日本語で難しいのは漢字です。
The hard part of Japanese is the kanji.
遅れたのは電車が止まったからです。
The reason I was late is that the train stopped.