"This is a nice place."
Translation:यह एक अच्छी जगह है।
I was under that impression too.
I was going with the assumption that you should use एक where you could use "one" and "a/an" fairly interchangeably ("I have two cats and a/one dog", "I drink a/one cup of coffee in the morning"), but skipped एक when "one" would sound funny (nobody would say, e.g., "We have to get you to one hospital!" or "One cat is one animal.)
I guess you could use "one" in this sentence, but only if you're sounding shocked: "whoa, this is one niiiiice place!"