のは～だ

のは～だ moves the piece of information you want to stress to the end of the sentence: "what I drink is coffee", "it was Osaka where he was born". The clause before のは ends in the plain form, and the stressed part goes before だ or です.

Compare わたくし私はまいあさ毎朝こうひいコーヒーをのむ飲む, "I drink coffee every morning". The facts are the same, but the のは version makes コーヒー the answer, as if to "what do you drink?".

A な-adjective before のは takes な, not だ.

• × 好きだのは音楽だ → 好きなのは音楽だ

The が or を that the stressed word had in the plain sentence drops out before だ.

• × 私が飲むのはコーヒーをだ → 私が飲むのはコーヒーだ

Giving the reason

With からだ at the end, the pattern gives the reason for something both speakers already know about: "the reason … is that …".

のは～だ or のだ

The explanatory のだ, or んだ, is a different pattern. It has no は and adds "the thing is" to a whole sentence. ちこく遅刻したんです explains what happened; ちこく遅刻したのはわたくし私です picks out who was late.