>>15
The auxiliary verb masu can be dropped without any additional word. Even in this case, it should be needed to conjugate the preceding word.
Probably the auxiliary verb desu cannot be left out without to add da or other; in some cases, that alternative word is unnecessary. I can't give an explantion of that pattern properly because of complexity.
Roughly speaking, if the preceding word is a noun, a pronoun or other substantive, da or something appropriate should be put, or the sentence gets irregular. Otherwise, if that word is an adjective or a main verb with/without the particle no, desu can be taken away instead of replacing. I can't explain any further...
For example :
お言葉ですが、お兄様 : The original sentence. The preceding word is a noun.
お言葉が、お兄様 : Strange. This means: "A word is my dear brother."
お言葉だが、お兄様 : OK. This means: "With all due respect, my dear brother."
かわいいネコちゃんですね : A noun and its suffix.
かわいいネコちゃんね : Maybe this is ok; lol, you might be called gay. This is the women's language!
かわいいネコちゃんだね : Right.
かわいいネコちゃん : This is sometimes ok but not enough.
ネコちゃんはかわいいです : An adjective.
ネコちゃんはかわいい : Correct.
ネコちゃんはかわいいのだ : Right, too. In such case, the particle no is needed besides da.
我々天狗は、幻想郷をずっと見守って来たのです : A verb.
我々天狗は、幻想郷をずっと見守って来たの : This is also the women's language and not complete.
我々天狗は、幻想郷をずっと見守って来たのだ : Good.
心の種が生まれそうです : An auxiliary verb.
心の種が生まれそう : Also ok but this is not perfect.
心の種が生まれそうだ : Correct.
いけない子なのです : An auxiliary verb and the particle no.
いけない子なの :This should be ok; Unless you are a woman nor.... This is the women's language too and not enough.
いけない子なのだ : Right.
こうですか？分かりません＞＜ : An adverb.
こうか？分からない＞＜ : Correct.
こうなのか？分からない＞＜ : Also ok.
By the way, Suiseiseki speaks desu too much and her usage is pretty odd like her eyes.