いい (good) which is an い adjective, is irregular, but only in certain cases.

For example, in the present affirmative polite, いい will be いいです

However in the present negative polite, いい will be よくありません

Similarly, in the past affirmative polite,  いい will be よかったです

And the past negative polite, it will be よくありませんでした

When using い adjectives, you can place the adjective before the noun:

これは　いい　コーヒーです = This is good coffee.

(これ = this. は = subject marking particle.　いい = good　コーヒー = coffee. です = is.)

You can also place an い adjective after the noun it modifies:

その　みずは　つめたいです = That water is cold.

(その = that.　みず = water. は = subject marking particle.　つめたい = cold. です = is.)

Like with い adjectives, you place な adjectives before nouns:

げんきな　ひとです = [He/She’s] a lively/energetic person

(げんきな = lively/energetic.　ひと = person. です = is.)

However, when な adjectives come after a noun, leave off the な:

かれは　びんぼうです = He’s poor

(かれ = he. は = subject marking particle.　びんぼう = poor. です = is. )

な adjectives are considered quasi adjectival nouns and that’s why the な is left off after the noun.

When you have a situation like this, how do you know whether the adjective is an い or な adjective?

はなは　きれいです = The flower is pretty

(はな = flower. は = subject marking particle.　きれい = pretty. です = is.)

The answer is that you just have to memorize that いれい(な) is a な adjective. Luckily there aren’t that many adjectives that are ambiguous as いれい(な).