重（おも）いんです。

バスは遅（おそ）いのです。

いいんですか。

赤（あか）がいいんですか。

いいのですか。

これがいいのですか。

川（かわ）があるんです。

覚（おぼ）えるんです。

ここはいいのですか。

ここは悪（わる）いのですか。

鼻（はな）が高（たか）いんです。

忘（わす）れるのです。

～んです/～のです- Japanese Grammar 

<div class='writeup-body'>のです is a construction with several different forms, each meaning roughly the same thing. This expression behaves similarly to だ and です by themselves (asserting that something is true). However, the addition of の strengthens the relationship being highlighted. Due to this, it will come across a bit like 'it is a fact that (A)' in English.<ul class='writeup-examples--holder'><li class='writeup-example'><div class='writeup-example--japanese'>ここのコーヒーが美味（おい）しいんです。</div><div class='writeup-example--english'>The coffee here is really good. (Strong assertion)</div></li><li class='writeup-example'><div class='writeup-example--japanese'>私（わたし）が悪（わる）いのです。</div><div class='writeup-example--english'>I am the one to blame. (Strong assertion)</div></li></ul>In these sentences, we can think of the ん, or の as simply adding emphasis to the statement. Due to の adding emphasis, this expression will be heard very frequently in explanations.<div class='writeup-caution'><div class='caution-header'>Caution</div>There is no real difference between ん and の. Only の is considered 'correct Japanese', but ん evolved to make things easier to say. ん is the most common form in spoken Japanese, and the use of の could be considered a bit formal, or 'stiff'.</div>While both the だ and です forms are equally as natural, only the です form will be used if adding か to the end ～んですか. This construction shows that the speaker is looking for an explanation about something.<ul class='writeup-examples--holder'><li class='writeup-example'><div class='writeup-example--japanese'>あなたはこれが欲（ほ）しいんですか？</div><div class='writeup-example--english'>Is this what you want?</div></li><li class='writeup-example'><div class='writeup-example--japanese'>お腹（なか）が空（す）いたのですか？</div><div class='writeup-example--english'>Have you become hungry? (Are you hungry?)</div></li></ul></div>

Grammar Info

んだ・んです

Explanatory, Emphasis, The fact is