Thank you very much guys. It's very nice of you to help me :)

Apostrophe s always clarifies possession. Using a different example: It was a painting of Tom = the image in the painting represented Tom. It was Tom's image. It was a painting of Tom's = Tom either created or owned the painting. It isn't really Tom in the painting, but the image represents the idea of Tom. If Tom was an inventor and I painted a picture of his idea, then we have a picture of an idea of Tom's. But most of us would say that the picture represented Tom's idea. It was a painting of Tom and Sarah = it was a painting of them both. It was a painting of Tom and Sarah's = they owned it jointly. We'd say: I saw the colour of Sarah's eyes. We could say: I saw the colour of the eyes of Sarah. The apostrophe is dropped, the eyes could only belong to Sarah so possession is clear. However, I'd always say Sarah's eyes.

I don't know the exact reason. English is funny like that. Using "of" is always more formal. Sometimes the formality is something that is 'archaic' and not used normal. It was a good idea of Tom's ... this is correct. Tom's idea was good... this is also correct. In English there are two ways to say the same thing... active voice and passive voice. In active voice the subject comes first. The subject is the main actor in a sentence. Tom is the subject. The object is the thing that is acted upon. In this sentence it is the idea. Active voice: Tom's idea was good. Passive voice: It was a good idea of Tom. Active voice: Sarah's eyes melted his heart. Active voice: Her eyes melted his heart. Passive voice: His heart was melted by the eyes of Sarah. Passive voice: His hear was melted by Sarah's eyes. Sorry if that makes it more confusing. Always try to use the active voice if you can. It is easier to understand.

I can imagine someone saying "I was a good idea of Tom's", perhaps it might be a difference in British and US English Bill. For example: Person 1: "Isn't this party great. I had no idea you were going to throw this surprise (party)" Person 2:"Yes it was a good idea of Tom's, wasn't it?" To try and answer your question Rocio I think it might be to do with the fact that the eyes cannot be physically separate from Sarah they are part of her, but the idea comes from Tom and Tom gets the credit for having the idea but the idea is separate from him, thus you can use "the idea of Tom's" or "Tom's idea". However I am not an English grammar teacher.

I could imagine someone saying "It was a good idea by Tom to evacuate the building" when recounting the day's events for example. Also you would say "I didn't understand anything".