I wished this post wasn't necessary, but sadly it is. It's actually not even a problem that most language learners that learn German as a second or third language struggle with. It's the natives - or at least some of them that seem to have problems with it.
The difference between "wie" and "als".
It's one of my pet peeves and it gets very tiresome once one of your friends insists on using those simple words the wrong way.
Let's find out the right way to use them first.
In this instance, both words are used to make comparisons. Let's start with "wie".
"Wie" means "as"; well, actually we're using "so ... wie", which means "as... as" here. I'll give you an example so you know what exactly I'm talking about:
"This tree is as tall as a house." (English) yeah, I guess you noticed ;-)
"Dieser Baum ist so groß wie ein Haus." (German)
(So "so ... wie" is used to show that one aspect of two things is similar. There's no need to further explain the sentence since the sentence structure of both sentences is identical here.)
as dark as - so dunkel wie
as sweet as - so süß wie (sweet can be used same as in english, so it can be about taste, but it can also describe "sweet" people for example)
as greedy as - so gierig wie
Difficult? Nah, I don't think so.
Let's take a closer look at "als" now.
In this case, "als" simply means "than" and is also used in comparisons. Is it more complicated?
Well, see for yourself.
I am taller than you. (English)
Ich bin größer als du. (German)
(Once again the structure in English and German is similar.)
"so ... wie" - is used to describe similarities"als" - is used to describe differences
(this does not count for negated sentences of course)
"Ich bin größer wie du."
that's how. (correct versions would be: "Ich bin so groß wie du." or "Ich bin größer als du.")