Everyone has moments like that. Without missing things like that, it would be almost impossible to become fluent in any language (even your native one), because it shows that you are just matching larger words with concepts (like rainbow for you did not equal rain + bow, but simply rainbow = coloured stripes in the sky). Other examples I've heard are: One girl I know didn't realise that pop corn came from corn This guy I know who has an IQ of about 150 didn't realise percent was PER CENT (as in, "in every 100"), and worse, he did Latin at school so cent = 100 was more than just general knowledge for him And my own one, I was about 25 before I realised that cigarette was like a cigar, but smaller, hence, cigar-ette.
This is why I love learning languages. It teaches me so much about my own, English. English is a weird animal, with a Germanic base from the Anglo-Saxons who came from near Scandinavia. Add in a heavy dose of Danish from the Viking Danelaw. Then overlay all that with French, a Romance language with a completely different structure. Of course, these were also Vikings, but they had adopted French by the time they invaded England.
I studied French for years, but now it is fascinating to compare Swedish to English and see the Old Norse connections and Germanic base.
Ser du den dubbla regnbågen? Vad betyder det/den?
Since regnbåge is an en word in the definite, the adjective needs to be dubbla and it also needs to be preceded by the front article.
In the second sentence, you can use either den (if you said 'it' in English and you're wondering 'What does the rainbow mean?' or det (if you said 'that' in English and you're wondering 'What does it mean that I saw a rainbow?')