I think な-adjectives cannot be used as complements in SVC type of sentences. (○) この　はなは　きれい （kono hanawa kirei） = This flower is beautiful. (○) この　きれいな　はな （kono kireina hana） = This beautiful flower (×) この　はなは　きれいな （kono hanawa kireina）

This is a great question, and I am sure many Japanese learners are confused. Here is my opinion. い adjectives are basic structures of adjectives in Japanese. When you write them with kanjis and hiragana, い adjectives tend to have both kanjis and hiraganas, which the hiragana part is called okurigana but you do not have to memorize the term. If you know kun yomi and on yomi, it is more easy to explain. The kanjis used in い adjectives usually have kun yomi. And grammatically, い parts can be conjugated. For example, the negation of 美しい(utsukusii, beautiful) will be 美しくない. On the other hand, the structure of な adjectives are different. Most of な adjectives can function as nouns without な. For example, 綺麗(kirei, beautiful)な, 素敵(suteki, great)な and 巨大(kyodai, enormous)な can function as nouns without な. 綺麗 is a noun that expresses a beautiful condition. 素敵 and 巨大 are same as this. They are nouns to illustrate certain conditions. If an adjective word can stand by itself without な part, it is a な adjective. Except な parts, な adjectives cannot be conjugated. The word きれい(kirei) ends with い, but this is an example of a な adjective. The negation of this word will be きれいではない. If I apply the conjugation of い adjective to きれい, it is going to be きれくない; however this is grammatically incorrect. I hope this explanation helps you.

What are you talking about? i adjectives always end with i. na adjectives on the other hand don't end with i. Unless you're referring to some irregular cases like "kirei", but let's be honest here, in kanji form, kirei is 綺麗, now there is no i at the end. PS: kirei is the only word that I can think off the top of my head for now, I'm sure there are A FEW more. And if you can't even bother remembering those when you eventually come across them then I don't know why you want to learn the whole language in the first place.