The verbal suffix ～ない is the semantic equivalent of the verbal suffix ～ません, but ～ない is the plain form, while ～ません is the polite equivalent. We can add ～です to plain form verbs to make them polite… or at least that’s what your textbook should’ve told you. Most simply:

tabenai desu. means the same thing as tabemasen.

ikanai desu. means the same thing as ikimasen.

Now is when I start splitting hairs. Please make sure you’re comfortable with the explanation so far before continuing.

We use the polite form to be, well, polite. Adding ～ない can sound a little bit like forgetting to say “please,” adding it at the end of a sentence. “Stop that … please,” is not quite as polite as “Please stop that,” as long as voice and body language are the same. So you do have the “hurdle” in Japanese to be able to say the polite form first, earliest, most easily when the situation calls for you to be polite. And please remember, Japanese is very dependent on situation, so it’s much safer to err on the side of politeness.

Here’s another hair to split, so please make sure you’re on board with that last bit before continuing.

One of my favorite things about Japanese is の- I think I must use の hundreds of times every day. If you’ve learned that の can turn any plain verb into a noun, you’ll understand quickly how cool this is. The plain verb “swim” is oyogu, so “swimming” can be rendered oyogu-no. 泳ぐのが好きです。Oyogu no ga suki desu.＝ “I like swimming.”

In the same way, you can turn an entire phrase or sentence into a noun, and this has an unspoken effect, when suffixed with ～です, of adding up to the message, “I’m explaining this to you.” But rather than being condescending, it’s (often) more like, “Well, you see, this is the case.”

東京に行きません。 I’m not going to Tokyo. (polite)

東京に行かないです。I’m not going to Tokyo. (polite, but…)

東京に行かないのです。The thing is, I’m not going to Tokyo. (polite)

Now then, this の often gets further shortened to ん . So if you say 東京に行かないんです。you can offer whatever body language that suits the situation: apology, defiance, resignation, whatever, and your listener will have social pressure to say something like, “I see. So you’re not going.” Sprinkling your J speech with ～んです will make you sound more natural, more confident and more adult all at one fell swoop. But please don’t forget to be polite when it’s called for. Please!

Third hair to split, but this should be intuitive:

Some sentences are said specifically for their polite content, for example Sumimasen. or O-negai shimasu. You should learn these sentences as they are, and never even consider how to make them simpler. I once tried Sumanai desu on my wife, and she laughed at me so hard that it became a family joke. Fair warning: if you try this on a J friend, you may not be so lucky.

Fourth and final hair to split today:

Using the simple negative verb plus ～です is what you can often expect from sumo rikishi. However, they tend to drop the で and sort of hiss the s in す. It’s fun to listen to the first interviews with non-J rikishi, because their two most common answers are So-ss. and Wakaranai-ss. They’re carefully counseled never to contradict the interviewer, so their answers are “That’s right,” and “I’m not sure.” They’re having the same problems as anyone learning Japanese in Japan, and little by little, you might suspect that the “I’m not sure” answer is really saying, “I’m not sure what your question means.”