Learn Swedish: Jag laddade ner filen igår. - I downloaded the file yesterday.

Because ladda ner is a particle verb, the stress falls heavily on the particle ner, not on the verb laddade. English speakers naturally want to stress the verb, but in Swedish, emphasizing the particle is crucial for sounding natural.

Can I separate laddade and ner by putting the object in the middle?

No. In Swedish, the verb and its particle generally stay tightly together and come before the object. You must say laddade ner filen or laddade ner den (downloaded it). You cannot say jag laddade filen ner.

I sometimes see this written as laddade ned. Is there a difference in meaning?

Ned is simply the older, slightly more formal spelling of ner. They mean exactly the same thing. In modern everyday Swedish, both spoken and written, ner is much more common, but you will still see ned in formal texts or older software interfaces.

If ladda ner means to download, how do I say 'to upload'?

You simply change the particle to upp, making it ladda upp. For example, 'I uploaded the file' is Jag laddade upp filen. Just like with downloading, remember to put the spoken stress on the particle (upp).