但凡【dànfán】

但凡 opens a clause and sweeps across every case: "anyone who", "whenever". The main clause takes 都 or 就, sometimes 总. It sits between neutral and written, and is common in essays and opinion pieces.

"If there were any … at all"

With 有 and a small amount – 一点, 一线, 一丝 – or with a verb plus 一点, 但凡 becomes a condition the speaker knows wasn't met: "if there were any way at all". The main clause says what would have happened, often with 也不会 or 就不会. The sentence is an apology or a complaint rather than a general statement.

The main clause needs 都 or 就

In the "anyone who" use, the main clause needs 都, 就 or 总 before the verb.

• × 但凡认识他的人说他好 → 但凡认识他的人都说他好

凡是 does the "every" job in plainer prose and has its own point. It has no "if at all" reading.