のでしょうか

のでしょうか asks a question politely and a little indirectly: you are asking for an explanation, or for something you can't work out yourself. Verbs and い-adjectives take it in the plain form, past or present; a noun or a な-adjective takes な.

• × 安全だのでしょうか → 安全なのでしょうか

のでしょうか or ですか

A plain ですか question asks for a fact. のでしょうか asks more gently, as if the listener might not know either, so it suits a question to someone above you, a stranger, or a sensitive topic.

Wondering out loud

To an audience, or to nobody in particular, のでしょうか raises a question without expecting an answer. Talks, articles and TV presenters use it to introduce a topic.

Other forms

In speech の often shortens to ん: んでしょうか. The plain form のだろうか is not the casual version; it is a question to yourself, mostly in writing. Among friends the same question is just の？ or のかな.