Not an easy question to answer. There are some cases that you can switch the words, and the meaning of the sentence will be the same and you won’t be mistaken. For example, you can say נשמה טובה and you can also say נפש טובה, (but נשמה טובה is used much more). On the other hand there are cases that you can’t switch the words. For example the sentence “נשמתו בגן עדן” (his soul is in haven) can not be used with the word נפשו. The word נפש has 2 meanings: 1) The believes, thoughts, feelings etc’ of a person. I geuss that you can say anything that is not the physical body of the person and makes a person who he is. 2) Whole person. You can say for example – “in this house there are 5 נפשות”, but it is very rarely used this way. The word נשמה is more “spiritual” word. You can talk about נשמה of some that is not alive any more. For example when you’ll translate the sentence “may god have mercy on his soul”, you have to use the word נשמה. Adi