"She runs fast."
Translation:Hon springer fort.
Can "springa" be used here? Maybe I don't quite fully understand what it means to be "infinitive", but I believe it to be when something is conceptual rather than actually happening, and the sentence seems to me to be able to be used in a conceptual way. "Oh, her? Yeah, she runs fast." Where am I wrong? Maybe my idea would be "Hon kan springa fort"?
Your comment is old now so I figure you have likely worked this out by now but maybe someone else may have your question as well.
The main verb in a standard sentence must indicate time (the finite verb)... ie be in the present, past etc tense. In this instance "Hon springer fort". An infinitive verb does not indicate time (hence infinitive verb) and is therefore used following the main sentence verb (that does indicate time). So you are correct that the sentence Hon kan springa fort functions properly. Where kan works as the main (finite) verb and springa follows in the infinitive form.
You can also use infinitives in the conceptual sense you are referring to. eg. Att springa är kul - To run is fun. But note that in that instance, where you are referring to running as a concept, the infinitive is not functioning as the verb but rather the subject of the sentence. The verb in that sentence is instead är.
Not sure if my explanation is particularly clear but hope it helps someone.
Arnauti does a much better job of discussing word order in his post here: https://www.duolingo.com/comment/8970470
and here: https://www.duolingo.com/comment/7202893