田中さんは何を見た↑。 ⇒Ta-form, Plain⇒"見た" is a plain-form, No. You don't need for "のだ".
田中さんは何を見たのですか。⇒Ta-form, Polite"見た" is a plain-form. To make this a polite form, Need foryou need "のだ(のです)"".
田中さんが見たのは何↑。 ⇒Noun⇒This is a noun sentence, Plainand "何" is a plain-form, No. You don't need for "のだ".
田中さんが見たのは何ですか。⇒NounThis is a noun sentence, Politeand "何" is a plain-form. To make this a polite form, Need foryou need "のだ(です)"".
In the Plain-form, "のだ" isor "です" are not necessary, while in the polite form, "のだ" isor "です" are necessary.
だれが来ますか。 ⇒Masu-form, Polite-form, No need for "のだ".
だれが来ましたか。⇒Masu-form, Polite-form, No need for "のだ".
だれが来ます↑。 ⇒Masu-form, Polite-form, No need for "のだ".
だれが来ました↑。 ⇒Masu-form, Polite-form, No need for "のだ".
だれが来る↑。 ⇒Root-form, Plain-form, No need for "のだ".
だれが来た↑。 ⇒Ta -form, Plain-form, No need for "のだ".
The word "Masu" implies politeness. so you don't need "のだ".