Pleas can someone help me with the "Distribution of Categorical terms". I've read the text about it but still I can't understand the when a proposition refers to all the elements in a class and when it doesn't.

Eg.
1. All S are P.
Here they say that S is distributed and P is not. This one is clear since they are talking about all of the S but about a fraction (a small part) of P.

2. No S are P.
Now this one drives me crazy. They say here both S and P are Distributed. I can see why S is distributed. But to me here P again seems Undistributed. Eg. If we state a concrete example such as "All cows are not mammals". Now here I can't understand how This proposition refers to all the elements of the Predicate "mammals".

3. Some S are P.
This one is clear to me. Both remain undistributed.

4. Some S are not P.
Here I have the same problem as that in "2".

Your example for number 2 is slightly different from the form given. Try this example: "No lizards are mammals." This tells you something about every lizard, i.e. that there is no single lizard anywhere that is a mammal. However, it also tells you something about every mammal (there is no mammal anywhere that is a lizard). This is because "No S are P" and "No P are S" are interchangeable.

I'm not sure about number 4. I would say that 3 and 4 are identical, with both remaining undistributed.