Al and del together

Question

Este profesor es diferente al del otro díaThis has al and del appearing right next each other. How does that work?

Accepted Answer

Hola Dhruv, 
"el de" means "the one", but here you have also "a" because you need to say "different to...", and you also have "del" because the "de" is linked to "el otro día" which makes it contract to "del", so you get "al del".
When we omit nouns you get "el de, la de, los de, las de". We could have said this sentence repeating the noun "profesor" but omitted it:
Este profesor es diferente al [profesor] del otro día.
This teacher is different to the teacher from the other day.
Este profesor es diferente al del otro día.
This teacher is different to the one from the other day.
Here is an advance lesson on this: 
el de/la de/los de/ las de
I hope it helps.
Inma