I agree with Umbran.
The thing is that the DM has a clear picture of the scene in his head, plus all the relevant background information, all the appropriate assumptions, and anything else that is needed.
The players have what the DM tells them, and nothing more.
Communication is
hard. At my place of work, I'm regarded as one of our most effective communicators, especially in the written form. Yet I've lost count of the number of times I've written something that seemed completely clear and obvious to me, only to have it come up in the review because my colleagues have misunderstood it - often because I assumed some other knowledge that was lurking in the back of my mind, but which I hadn't (and should have) mentioned.
It's not about blame. Ultimately, it doesn't matter if the DM communicated badly or if the players listened badly - in the scenario described, the game may be about to go off the rails because the players are working with incorrect information. Since the players
can't know they've gone wrong (or else they would have fixed it!), it falls to the DM to correct the situation.
And, to finish up, a few things I've learned that help with getting the message across:
- Don't
imply things. If there's something the players should know,
tell them.
- Don't swamp the players with detail. If you talk for more than about 20 seconds, by the end the players will likely have forgotten the details from the start. That's not because they're stupid, or not listening, it's because short-term memory is limited, especially if it's not clear what is and is not important.
- Watch your language. Your players aren't Gygax, or Tolkien, or even REH. Chances are that they won't know exactly the same set of words as you (and this is even more true if they're playing in a second language, of course). If you throw in lots of obscure words into your descriptions, you're as likely to muddy up your descriptions as you are to evoke an old-timey feel.
This last one is controversial, of course, since descriptions should
also be evocative, and choice of words is an important part of that. Here, I've found a hybrid approach works quite well - describe the parameters of the situation in almost technical terms ("three goblins, here, here and here..."), but then add some limited flavour on top, generally in peripheral matters ("the lead goblin capers and cackles with a malevolent glee...").
(Of course, if your players are Gygax, Tolkien and REH, or equivalently skilled with the language, you should ignore the above. But then, you'll have realised that.

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