When children are learning the lower case alphabet, they frequently mix up certain pairs of letters: p and q, g and q, l and I, and especially b and d. This is normal. As they get experience, they recognize differences in these letter pairs. But mix-ups with b and d might linger well into elementary school.
One solution is to tell the child that b and d look at each other. Draw the letters with the b loop facing the d loop, and put dark irises in the loops. Tell the child that b comes before d in the alphabet, so when looking at b d, b is the first letter and d is the second letter.
Another way to handle the b d problem is to have the child make fists with both hands while holding up the thumbs. When the child looks at his left hand, it looks like the letter b with the thumb the stem and the fist the loop of the letter. When the child looks at his right hand, it looks like the letter d. Now tell the child to bring her fists together until they touch and to look at the shape. Her hands should look like a bed with the thumbs the bedposts and the fists the mattress. If the child knows the word “bed,” the child can easily figure out b and d.