This is one of the shortest lessons in IMABI. So, feel relaxed as you go through more information about the copula in Japanese.
である is quite ceremonious and normal in writing, but when it's spoken, it's forceful by presenting things as fact. A funny example is the title of a book by the famous writer 漱石.
I am a cat.
Word Note: わがはい is a very pompous, old-fashioned pronoun, and it is well-known from this title. Aside from this title, never use this pronoun. No one uses it. We just need some comic relief.
So, that Kimishima guy is someone from that company, no?
From 顔に降りかかる雨 by 桐野夏生.
Grammar Note: The reason why this example is cited is because of a peculiar avoidance of である due to formality. The 連体形 of the copula can be expressed in one of two ways depending on the situation: である and な. We know that the latter is used with 形容動詞. For instance, you say ばかな人 for "stupid person," although ばかである人 is still grammatically correct.
When you wish to use something like んだ・のだ, which is for explicit emphasis, to "nominalize" an entire phrase, な is obligatory after a noun if it is the final word. If you have それは魚 and want to add んだ, you say それは魚なんだ. In this example, two regular nouns 人 and わけ find な in between him. It's clear in context that [そっちの会社の人] is modifying わけ. Yet, わけ is being treated a lot like の in のだ. Thus, we have yet another instance of な being more appropriate than である.
だ & です
As far as usage is concerned, both だ and です make a declarative sentence. Also, both can act as a final particle to strengthen an appeal to the listener.
Japanese is easy.
Tomorrow is a break.
Note: This sentence can be interpreted differently depending on context. It could show that you want to drink iced tea, brought iced tea, etc.
Grammar Note: The omission of the copula is allowed, but the statement loses some of its assertiveness, which could be helpful to not sound rude or blunt in the case of だ.
Well, it's homework (time).
Well, it's (time to) depart.
Grammar Note: Here, the copula is used as a filler word. The same can be done with です.
Culture Note: あっかんべー is where you stick out your tongue while pulling down your eyelid.
です can make some conjugations polite whereas だ doesn't! だ is plain and the basic form of adjectives and verbs are already plain.
12a. 簡単ではないだ X
12b. 簡単ではないです。 〇
It's not easy.