Ich wusste es von Anfang an.

Spanish Translation

Lo supe desde el principio.

What is the function of an?

I think it serves the same function as “on” would in an equivalent English sentence: “I knew it from that point on”, which I believe is a shortening of “onwards”: “I knew it from that point and onwards”. You can drop it in modern English, and just say “I knew it from the beginning / from the start” .The word “from” there kind of implies “and onwards”.

I’m a little surprised that it isn’t “vom” in German… I would expect “Ich wusste es vom (von dem) Anfang an”.


Good observation with the dative!
“von Anfang an” is kind of a set phrase. I can’t actually tell you why it’s not “vom”.

“vom Anfang an” also exists, but I think the difference here is that with “vom”, you’re talking about a concrete, tangible beginning, while with “von”, it’s an abstract beginning, like it is here in this sentence.

You could for example say “Lies das Buch noch mal vom Anfang an!”, and you’re talking about the beginning of the book.

“von Anfang an” often really means “it’s always been like this”

Here is a discussion about the difference:
von oder vom Anfang an... ???? - Sprachlabor: Englisch ⇔ Deutsch Forum - leo.org

One post agrees with my view:

Beides ist möglich. Wird Anfang als Abstraktum verwendet, dann von Anfang an.
Wird auf einen konkreten Anfang Bezug genommen, kann auch vom Anfang (des Projekts) an verwendet werden.