のやら / ものやら / ことやら

のやら, ものやら and ことやら end a question nobody can answer: "I wonder", "who knows". They follow the plain form, usually with a question word earlier in the clause, and the sentence either stops there or goes on to わからない or けんとう見当もつかない.

A な-adjective takes な: どれほどたいへん大変なことやら, "who knows how hard it is".

のやら, ものやら or ことやら

The three overlap, and のやら is the most general. ことやら sighs over what will become of something, most often in どうなることやら and いつになることやら. ものやら, often after a た-form, is at a loss over what to do.

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These are musings, said half to yourself. They sit naturally in narration and essays, and in conversation they sound wry or a little old-fashioned. The everyday way to wonder aloud is かな or のだろう.

Two のやら in a row, as in うれしい嬉しいのやらかなしい悲しいのやら, is a related pattern with its own entry: "whether … or …, I can't tell".