Japanese adjectives split into two types, and they conjugate in completely different ways. The label comes from how they attach to a noun.
i-adjectives
These end in い and conjugate by themselves, no helper word needed. 高い (takai, expensive or tall) attaches straight to a noun: 高い本 (takai hon, an expensive book).
takai to takakunai, not expensive
takai to takakatta, was expensive
na-adjectives
These need な to attach to a noun and use です or だった for tense. 静か (shizuka, quiet) becomes 静かな部屋 (shizuka na heya, a quiet room).
shizuka na heya, a quiet room
shizuka ja nai, not quiet
The trap
Some na-adjectives end in い and look like i-adjectives. They are not. The common ones to memorise are きれい (kirei, pretty or clean), 嫌い (kirai, disliked), and 有名 (yuumei, famous). きれい takes な: きれいな花 (kirei na hana, a pretty flower), never きれい花.
Quick test
Put the word in front of a noun. If it connects directly, it is an i-adjective. If it needs な, it is a na-adjective. The two exceptions above are worth drilling until they are automatic.
Test one of these examples in the translator and see how it comes out.
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