This is just going to be one of those strange rules that you have to memorize, but だ will never snap onto an i-adjective.

What’s an i-adjective? Well, it’s any adjective in Japanese that has the hiragana character い (i) attached to the end of it. In other words, all of these adjectives that you saw earlier:

It’s totally chill to snap です onto i-adjectives, which is what we do for formal-sounding sentences. But in casual Japanese, when you make an “A is B” construction where B is an i-adjective, you don’t need to put だ at the end of your sentence.

Let's look at a delicious example...

A = ピザ / piza = “pizza"
B = 美味しい / oishii = “delicious; tasty; good”

FORMAL:
◯ ピザは美味しいです。
◯ ピザ　は　おいしい　です。
◯ Pizza is delicious.

CASUAL:
◯ ピザは美味しい。
◯ ピザ　は　おいしい。
◯ Pizza is delicious.

EVIL, DISGUSTING, & WRONG:
✕ ピザは美味しいだ。
✕ ピザ　は　おいしい　だ。

While this new rule might be scaring some of you, this is actually really good news, because it means that we can express full sentences using even fewer words than before. All we need is “NOUN + i-adjective.”

For example, here are some nouns:

(Note: I normally wouldn't provide hiragana readings of katakana words, but I suspect that some of you will still be having trouble with katakana at this point in your studies.)

Assuming you've been keeping up with your kana studies, I'm guessing it took you about 48 seconds to memorize those words.

And now you can use them to make all of these sentences...

ドアは大きい。
ドア　は　おおきい。
The door is big.

パンは安い。
パン　は　やすい。
Bread is cheap.

テーブルは小さい。
テーブル　は　ちいさい。
The table is small.

スマホは高い。
スマホ　は　たかい。
Smartphones are expensive.

ケーキは大きい。
ケーキ　は　おおきい。
The cake is big.

チーズケーキはまずい。
チーズケーキ　は　まずい。
The cheesecake is bad (i.e. “it tastes bad”).

(Note: Since we have no context for these phrases, I'm just guessing when I choose to make them specific/singular [e.g. "The cake is big."] and not general/plural [e.g. "Cakes are big."]. Translations would vary depending on the situations in which the phrases were appearing.)

I hope you’re starting to realize just how simple it is for us to form sentences in Japanese. Sure, you might mess up the nuances of these sentences, which can change, for example, depending on whether you use は, が, or even no particle. But that's the kind of high-level stuff you can worry about after you're conversing regularly in Japanese.

Also, I hope you're not getting burned out by all of these table and charts!