Learn Icelandic: Sjúklingurinn borðar minna í dag.

Because the sentence is making a comparison. lítið is the basic adverb for "little", but since the patient is eating "less" (compared to yesterday or earlier), we need the comparative form, which is minna.

Would minna change form if we were talking about a female patient or multiple patients?

No. In this sentence, minna is acting as an adverb because it describes the verb borðar (how much they eat). Unlike adjectives, adverbs do not change to match the gender or number of the subject. It will always be minna here.

How does the word Sjúklingurinn break down to mean "the patient"?

In Icelandic, the word for "the" attaches to the end of the noun instead of being a separate word. The base noun is sjúklingur (a patient), which is a masculine word. Adding the masculine definite article -inn to the end gives you Sjúklingurinn (the patient).

Why is "today" written as two words (í dag)?

Icelandic does not have a single word for "today". Instead, it uses the phrase í dag, which literally translates to "in day". The word dag is in the accusative case here because the preposition í usually takes the accusative when referring to a point in time.