First of all, "describe" is a common word, people use it all the time, and it is fairly basic vocabulary. "Ascribe" is a rare word used in special situations. ("Describe" is one of the 1,000 commonest words; "ascribe" isn't even in the top 5,000). So a simple strategy is "use 'describe' and forget 'ascribe.'" Wait until you actually see the word 'ascribe' in something you are reading, and look it up in a dictionary when you need to. Second, for an exam strategy, what you probably need to know is that to "describe" something means to "to make a picture of something by using words." For example, "Can you describe your house?" "Yes, it is a two-story wood-frame house with grey siding, dark grey roof shingles, and the house number 83 on it." And, probably all you need to know about "ascribe" is that it means something totally different! "Ascribe X to Y" means "to explain something about Y by saying that Y has X." For example, "scientists now ascribe consciousness to animals." That means "scientists now explain the behavior of animals by saying that animals have consciousness." Or, "the politician said he was acting in the public interest, but his rivals ascribed his actions to corruption."

To "describe" means to give another person, by the written or spoken word, a clear mental picture. Often we "describe" the DETAILS of our day to a friend by saying, " I woke up at 6. I ate a big breakfast. I walked to work". To "ascribe" basically means to give or assign CREDIT. You could say, "I ascribe my health to a good breakfast every day" or "I ascribe my good attendance at work to a good alarm clock". I "ascribe" my success at work to a good education. Hope this helps