Not too complicated, right? The inflected suffixes and prefixes work the same in both stems. The niphal only adds the נִ to the beginning of each word in the perfect, and changes the vowels a bit. You would do well to also note the middle dot in the first letter of the root for the niphal imperfect. While this does not affect the way we pronounce the word, it will at times help you identify it as a niphal imperfect.
Finally, those with keen eyes may have observed that the niphal perfect 3ms (“he was found”) is identical to the qal imperfect 1cp (“we will find”). As with other examples of identical forms we have seen, the intended meaning of the author must be discerned from the context.
Let’s take a look at these two stems of the word מָצָא in action.