Learn Icelandic: Af því að heilsan mín er slæm.

When someone asks a question starting with Af hverju (Why), you will almost always begin your answer with Af því að (Because). It is the standard way to give a reason in Icelandic.

Why does the sentence say heilsan mín instead of just heilsa mín?

In Icelandic, when you place a possessive word like mín (my) after a noun, that noun almost always needs the attached definite article (the word for "the"). Heilsa means "health", and adding -n makes it heilsan ("the health"). Literally, the sentence is saying "Because the health my is bad."

Why is the word for "my" spelled mín here instead of minn?

Possessive words have to match the gender of the noun they belong to. The noun heilsa is feminine. The masculine word for "my" is minn, but because heilsa is feminine, we must use the feminine form mín.

Why is the adjective for "bad" written as slæm instead of the dictionary form slæmur?

Just like the word for "my", adjectives must match the gender of the noun they describe. Because heilsa is a feminine noun, the adjective drops the masculine -ur ending and becomes the feminine form slæm.