I want to know why $\omega \neq \omega+1$. In Kunen's book, Set Theory,chapter I.7, he said: $1+\omega=\omega \neq \omega+1$.
I want to know why $\omega \neq \omega+1$. 
 A: $\omega + 1$ has a limit point (i.e. $\omega$ — using the von Neumann definition $\omega + 1 = \omega \cup \lbrace\omega\rbrace$) in the order topology while $\omega$ is discrete in the order topology.
A: Because the elements of $\omega$ are all finite, whereas $\omega + 1$ has one infinite element.
A: There is an easy way to see this. You need to apply the definition of ordinal addition:
$$\omega + 1  = \omega \times \{0\} \cup \{1\} \times \{1\} = \{0, 1, 2, \dots 1^\prime\}$$
So $\omega + 1$ has an element at the end that is not a successor of anything while $\omega$ does not.
On the other hand, $$1 + \omega  = \{1\} \times \{0\} \cup  \omega \times \{1\} = \{1 ^\prime, 0, 1, 2, \dots\} \cong \omega$$
so you see that addition doesn't commute.
There is some more information about this here on Wikipedia. Hope this helps.
A: I find pictures to help. The idea here is that $\omega$ is a limit ordinal and tacking on the ordinal $1$ after it is fundamentally different:


The picture for $\omega$ has a curved edge which indicates that it is a limit ordinal opposed to being a successor ordinal. When we tack on $1$ to the right of $\omega$ we have this ordinal $\omega+1$ that contains a limit ordinal which is not something that occurs in $\omega$. This means that $\omega$ and $\omega+1$ can't be isomorphic.
Can you use see why $1+\omega$ and $\omega+1$ aren't equal? Do you see why $1+\omega = \omega$?
