idea
The German word 'Idee' is used in the same way as the English word 'idea'. It refers to a thought, concept, or plan that is formed in the mind. For example, 'Ich habe eine Idee, wie wir das Problem lösen können' means 'I have an idea of how we can solve the problem'.
I really like the idea.
This phrase is used to express liking or approval of a particular idea. The verb 'gefällt' (like) is used in connection with 'mir' (me) to express personal preference or taste.
That's a good idea.
This sentence is used to compliment or show agreement with an idea. 'Das' (that) is used to point out the idea, 'ist'(is) signifies existence or occurrence and 'eine gute Idee' (a good idea) shows approval.
I have no idea.
This phrase is used when the speaker doesn't know the answer to a question or when they are clueless about a situation. 'Ich habe' (I have) used here indicates possession of knowledge, which in this case is non-existent hence 'keine' (no).