(1) The weather can change very quickly in the mountains. (2)The weather is nice now, but it could change later. Why can't we use "can" for the second sentence? Can we use "could" for the first sentence?
These can be confusing. :)
Along with several other related meanings, one of the uses of can is to express ability. So, the weather has the ability to change very quickly in the mountains. Could is also used to express ability (also along with several other related uses), but in the sense of possibility or likelihood of happening.
So, if you say that the weather is nice now, but can change later, you are simply saying that weather has the ability to change (as, of course, we all know). If we say that it could change later, then we are suggesting that there is a likelihood that it will change later over and above the weather's general ability to change.
These two sentences might do a better job of drawing the distinction:
We can have up to 10 people for dinner tonight.
We could have up to 10 people for dinner tonight.
The first sentence is saying that you can "put on" a dinner for 10 people tonight; you have a table that can accommodate 10 people, and that you have the necessary food drink, and cooking ability to make dinner for 10 people. It doesn't say anything about the possibility that you will put on such a dinner, just that you can.
The second sentence is saying that up to 10 people might show up for dinner tonight, implying perhaps that you have sent out 10 invitations to dinner tonight and expect that some or all of the people invited will come.
Here's a detailed explanation of some different uses of these two words, along with would.