Is it "a welcome-sign" or "a welcome
sign"? The way you stress the words I mean.
In this case, I assume you meant the former, which means it's a compound made up from two nouns, it's not an adjective modifying a noun, which you seem to constantly confuse it with. And yeah, it is a special kind of sign, one that tells you "welcome" rather than one you are happy to lay your eyes upon.
So, if it is indeed a compound, everything except the very last part stays the same. And that last part is conjugated just like it is if it were to stand on its own.
en skylt - skylten - skyltar - skyltarna
en välkomstskylt - välkomstskylten - välkomstskyltar - välkomstskyltarna
The tricky part is making the compound in the first place. For this word it's easy since you just stick the two nouns together, but the cognate of "warehouse", varuhus
, uses an old, frozen genitive (vara
), and "table cloth", bordsduk
is made up from noun + modern possessive clitic + noun. Or it is to our modern eyes anyway, depending on the noun in question, it may very well be the old frozen genitive that the modern clitic is based on