Learn German: Der Buchstabe A steht am Anfang. - The letter A is at the beginning.

While it is true that most German nouns ending in -e are feminine, der Buchstabe is an exception. It is a masculine noun. It also belongs to a special group of nouns (often called 'weak nouns' or N-declension) that add an -n in most other cases, such as the accusative (den Buchstaben).

The English translation says 'is', but the German sentence uses steht. Why not use ist?

You could technically use ist, but German strongly prefers positional verbs like stehen (to stand) or liegen (to lie) to describe exactly how something is located. Because a printed letter is seen as resting upright on a line of text, German describes it as 'standing' (steht).

Why is it am Anfang instead of an den Anfang?

The word an is a two-way preposition. Because the letter is already located at the beginning (answering 'where?'), we must use the dative case. Der Anfang (the beginning) becomes dem Anfang, and an + dem combines into am. If you were moving the letter to the beginning (answering 'to where?'), you would use the accusative case (an den Anfang).

Why do we use the preposition an for the beginning, rather than in?