In Japanese, there are exactly three い-adjectives which sometimes think they are a な-adjective when appearing in front of a noun. These adjecives are 大きい and 小さい, which mean big and small respectively, and おかしい, which means funny or strange.
When we write these adjectives in front of a noun, we can write them in two ways, as shown in the table below.
|A big car||大きい 車||大きな 車|
|A small car||小さい 車||小さな 車|
|A strange car||おかしい 車||おかしな 車|
This behavior can only occur in front of a noun. In all other cases, these adjectives are treated as い-adjectives only.