白日作梦 [- - zuò mènɡ]dream in broad daylight; daydream; be nothing but a daydream (for sb.); a pipe dream; dream with one's eyes open; dream daydreams; cherish a pipe [fond] dream; indulge in wishful thinking; an idiot's daydream:
•¶ 幻想在一天早晨就把外语学好， 那只是白日作梦。 It is simply a fantastic imagination to conceive that one can master a foreign language overnight.