Learn German: Der Apfel ist groß.

In German, every noun has a grammatical gender: masculine, feminine, or neuter. Apfel (apple) is a masculine noun. Because of this, it uses the masculine word for "the", which is der. There isn't always a logical reason why a noun has a certain gender, so it is best to always learn the article along with the noun.

Why is der Apfel in the "nominative" case?

The nominative case is the basic form of a noun used for the subject of a sentence. The subject is the person or thing doing the action or simply existing in a state. Here, der Apfel is the subject because it is the thing that "is" big. When you learn nouns with der, die, or das in your vocabulary lists, you are already learning them in the nominative case.

Why doesn't the word groß change to match the masculine noun?

When an adjective comes after a verb like ist (is), it doesn't get any special endings. It stays exactly the same whether the noun is masculine, feminine, or neuter. You only have to worry about adding endings to adjectives when they come directly in front of the noun (like if you were saying "the big apple").

What is the weird letter at the end of groß?

That letter is called an Eszett or scharfes S (sharp S). It is a special letter used in German that makes a sharp, hissing "s" sound, just like the "ss" in the English word "glass" or "cross". If you don't have a German keyboard, you can usually type it as a double "s" (gross).