Using な after い-adjectives

What does it mean to put a な after an い-adjective? For example, many Youtube videos are called begin "いいなCM", and a Japanese text book I have uses this sentence as an example:

この通りを行くと 大きな 交差点がある。

Go along this street, and you will find a large crossroads.

What is the meaning of this な? It is also unusual that the 大きい is missing the final い but the いいな is not. I cannot find it in my textbook or on using Google.

Answer

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  Your example いいな CM is not an adjective modifying a noun. It can be taken as a quoted sentence modifying a noun. It may be more recognizable if it were in quotes like "いいな" CM. The な in いいな is a sentence final particle that adds the first person's subjective feeling to the proposition.

  
  


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  There are both the i-adjective 大きい and the (possibly) na-adjective 大きな. However, whereas the i-adjective has full paradigm, the 大き part in the na-adjective form cannot be extracted and be used like 大きだ. It looks like the na-adjective form is a fossilized form. For this reason, traditional grammar reserves an independent category called 連体詞. The two words are usually interchangable, but for a subtle difference, see the answer here: i-adjectives used as na-adjectives: is there a difference? (e.g. 大きい versus 大きな).