@NNC: English

The English is fine, only the meaning is odd. In English a nightmare is a dream, only an unpleasant dream; and nightmares, like other dreams, are generally forgotten quickly; although the memory that it _was_ a nightmare may linger.

So it's (semantically, not grammatically) strange to remember a dream _as if_ it had been a nightmare. The last thing you forget about a dream is whether it was pleasant or not, i.e., whether or not it was a nightmare.

true -- and what if it said something like "as a terrible experience"? Because in Spanish "una pesadilla" may be a bad dream or just a bad experience, maybe if it's translated in this second sense, it would sound better in English.

Yes, implying he remembered the nightmare as if it had been a real experience. That makes sense.

Would it be OK now?

I like quibbling, but I can't find anything here to quibble about. In fact I very much like the suppression of the wordy "as if it had been," which adds nothing not already implied.

What do you think of the Spanish? Well expressed or a little flabby? What about "recordó el sueño como pesadilla espantosa"? Is that good Spanish? [I don't mean to suggest you change Pharamp's sentence.]

To be honest, I don't like much the original sentence, but as someone from Latinoamerica said that it was OK, I'm not going to say anything about changing it. But I'll try to add another translation into Spanish that sounds better to me.
By the way, it would be "como *una* pesadilla espantosa", but to me it would sound better (and simpler) "recordó el sueño como algo terrible".

Thanks for the tips.

Of course Tatoeba's ethos discourages needless changes to OK sentences, but some examples are better than others. Good grammar and naturalness top the list of desirable attributes; but clarity, factual accuracy, and style also count and are worth discussing; and sentences + translations are a great place to look for such universals of good style as economy.