Learn Swedish: Jag har inte matat någon surdeg idag. - I have not fed any sourdough today.

This is a perfect example of the "ingen-rule". In Swedish, you cannot use ingen, inget, or inga as an object if the clause has an auxiliary verb (like har) paired with a main verb (like matat). Because the verb phrase is split, the negation must also be split into inte ... någon.

Could I say Jag har matat ingen surdeg idag?

No, that is grammatically incorrect in Swedish and sounds very unnatural. English speakers often try to do this because it mirrors "I have fed no sourdough," but the Swedish rule strictly forbids using ingen after a verb in the supine form (matat). You must split it.

If I change this to the present tense ("I am not feeding"), do I still have to split the negation?

No! In the present tense, there is only one verb (matar), so there is no auxiliary verb getting in the way. Because of this, you can safely combine them and say Jag matar ingen surdeg idag. You can also say Jag matar inte någon surdeg idag, but using ingen is often considered more elegant when there's only one verb.

Why does surdeg take någon here instead of något?