Nominalise sub-sentence

昨日何を食べたか忘れた I forgot what I ate yesterday (I forgot about "what did I eat yesterday?").

彼は何を言ったか分からない I don't understand what he said (I don't understand about "what is he talking about?").

結婚しようかと思っている I am thinking about getting married

Also works with Question word. Often particles disappear.

どこか遠くに行く Going somewhere far away

この文章は何かが可笑しい As for this sentence, something is wrong with it.

いつか日本語に行きます I'll go to japan someday

Or

can denote 'A or B', as a summation.

京都か東京に行く Going to Kyoto or Tokyo

A combination of first a Or and then a Nominalise sub-sentence. Good exercise!

死ぬか生きかということだ It's a matter of living or dying.

やるかやらないか早く決めてください。 Whether you will do it or not do it, decide it quickly.

Combined with a question word and its Nominalisation is basically the same thing.

明日は雨かどうか分からない。 Whether it will rain tomorrow or whatever, I don't know.

Denotes a question. Note that this is mostly forformal speech, using it in casual makes it sound a bit forceful.

どう思いますか？どうおもう~~か~~？ What do you think?