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 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 school?
In casual conversation you can also 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.