Understanding what is said
Speaking portuguese has made me realise that what I understand depends on a lot more than just knowing and hearing the words.

I found myself in an interesting situation with a Brazilian friend recently where I understood everything that had been said in terms of the words, but at the end of the conversation, I realised that I had completely failed to pick up on the cultural issues, gestures, tone of voice etc. that was intended to give the conversation a completely different meaning.

This sounds obvious, but I do wonder how much this happens with our own language without us really realising it.