Different from vs. Different than

In most cases, the expression different than is considered nonstandard, even though it is very commonly used in American English:

my plan was different than his plan

my plan was different from his plan
mine was different from his
my plan differed from his

From is a preposition and should be used before a noun or pronoun. This is the most common sentence pattern, so different from is usually correct. Than is a conjunction, so it should only be used before a clause (a group of words with a subject and a verb), not before just a noun or pronoun:

his plan was different than I thought
saw things differently than I did

different from what I expected

In the last example, the pronoun what is the object of the preposition from, so from is correct.