@ankara-- Which class are you studying this for?
Yea, these terms get confused a lot because they're pretty similar. You already know from the article what bradypnea is, it's when the number of breaths per minute decreases.
You might have also heard of apnea before, it's usually talked about as "sleep apnea." That's when someone stops breathing momentarily and usually it's during sleep.
Hypopnea on the other hand is sort of the precursor to apnea, it's when breathing becomes shallow. Notice I'm not saying that the breathing is slower. Someone who has hypopnea will usually take normal number of breaths, it's just that the breathing will be shallow, less deep.
I hope these make more sense to you now.