It is incredibly easy to turn a statement into a question in Japanese—all you need to do is add the particle か to the verb.
山田さん は 先生 です。
Yamada-san is a teacher.
山田さん は 先生 ですか？
Is Yamada-san a teacher?
マイク は 水 を 飲みます。
Mike will drink water.
マイク は 水 を 飲みますか？
Will Mike drink water?
In formal writing, there is no need for the question mark (it never used to exist in Japanese) as か makes it clear that the sentence is a question; however, in most non-literary texts it will be in included.
If we change the verb to the negative then we can use か to make a suggestion or an invitation.
一緒 に 食事 を しませんか？
Won’t you have dinner together?
飲み会 に 来ませんか？
Won’t you come to the drinks?
We can also add か to the standard form of the verb (this structure has a key grammatical function, as we shall see later); however, this is rarely a structure used to ask a question—it is too blunt. Instead, you will see の or なの added, depending on the sentence ending.
の is used if the sentence ends in a verb or i-adjective
なの is used if the sentence ends in a na-adjective or noun
今、忙しいの？
Are you busy now?
その ビル は 学校なの？
Is that building a hospital school?
In casual conversation you can also put make the question in statement form and let a change in intonation do the rest, e.g. “You going?”
食べる？
Will you eat?
ジョン は 会計士？
Is John an accountant?
Note that you will never hear this way of questioning used with the copula—either standard or polite.