Let see this example:
The dictionary said:
so that/in order that somebody/something would: used for saying why somebody does something
and it didn't say the meaning of would.
Is would" the past form of "will"? and "will" is used to predict something.
and "could" the past form of "can"?
Ditto @matias, but let me add that the more common situation is that "would" indicates intention and "could" indicates ability. If you said, "I asked Bob for help but he couldn't help me", you are saying that Bob was unable to help you. Maybe he wanted to help you, maybe not, but it doesn't matter because he was not able. If you said, "I asked Bob for help but he wouldn't help me", you are saying that he was able to help but he refused.
In this case, would is used to express a likely situation, and could is used to express ability in the past.
You say "I hid the presents so that she couldn't find them" if you hid the presents in a place you were absolutely certain it was impossible she would find them in, and "I hid the presents so that she wouldn't find them" if you hid them in a place in which *you predicted at that moment she would not find them in, it was unlikely, yet it was possible.
The difference is of impossibility vs. unlikeliness.
Further information can be found on the Merriam Webster Learner's Dictonary, here, and here is a similar question, though not the same.
