"I dream about my girlfriend."
Translation:Eu sonho com a minha namorada.
I've found uses on-line of ‘sonhar’ with ‘com’, ‘em’, ‘de’ and ‘sobre’, in order of popularity. Each one is about ten times as popular as the next, except, interestingly, if you search only for Portuguese sites in which case ‘de’ is twice as popular as ‘em’.
But maybe I'm looking at this the wrong way, because the preposition used seems to depend on the dream. If it's ‘tu’ then virtually always ‘contigo’ is used. I've also tried various verbs and then it's mostly ‘de’. I've tried some nouns as well and they seem to want ‘com’. I've also tried to figure out if there are cases where ‘em’ or ‘sobre’ are preferred but couldn't find any.
I've punched quite some things in and I'm proposing this rule for beginners: if it's a (pro)noun, use ‘com’, if it's a verb, use ‘de’. Of course you can use the conjunction ‘que’ as well.
The correct answer seems so strange from an English perspective - to say "I dream WITH my girlfriend" when she is not there. Then what if you and your girlfriend dream together about opening a business, how do you say "I dream with my girlfriend" when she's not actually in your dreams and you are not dreaming about her?