I am also confused on this. Why isn't "bathroom tub" acceptable, instead of "bathtub"? ... especially when the sentence seems to be describing the location of the soap. Perhaps, "stanza di bagno" literally means bathroom, and "bagno" is generally short for this, and while "vasca" can mean "sink", it might usually refer to something larger (sink would usually be "lavello").
Here 'da' has a meaning like in 'camera da letto', meaning bedroom. In Italian, 'da' is also used when something is used to do another something. It can be used like 'to' in English in cases like this:This recipe is easy to prepare = Questa ricetta è facile da preparare. About vasca da bagno? It's simply kind of an idiomatic phrase you can use instead of mere bagno. p.s.: the verb, however, to have a shower, is: fare una doccia (e.g. faccio una doccia - i have a shower)
Here are some words I copied from Mukkapazza a while back from elsewhere on this site. Looking at this "vasco da bagno" puts the use of da as an adjective.
"Prepositions are one of those things that rarely translate directly from one language to another but here are a few pointers that might help:
da + article: usually means from (i soldi cadono dal cielo/money is falling from the sky) but in some instances can mean to (andiamo dalla nonna/we are going to grandma's) or make a word an adjective (sala da pranzo/dining room/dining hall)...
• da + la = dalla • da + il = dal • da + lo = dallo • da + le = dalle • da + i = dai • da + gli = dagli
di + article: usually means of (è il ristorante dell'uomo/it is the man's restaurant/it is the restaurant of the man) but can also mean from (di dove sei? where are you from?) or about (scrivo di te/I write about you)...
• di + la = della • di + il = del • di + lo = dello • di + le = delle • di + i = dei • di + gli = degli"
I don't think this is quite the full story eg the use of delle etc to mean "some" but it's a great help in getting one's head around di and da :)
I've put "The soap is on the bath in the bathroom". That "da bagno" made me all confused! I was trying to be too literate, and it didn't work. Oh, learning new languages it was so much easier when we were all kids! I'm brazilian btw so Italian shouldn't be a massive problem, although the prepositions get me all the time.