Learn French: L'infirmier travaille à l'hôpital. - The nurse works at the hospital.

You would change the subject to l'infirmière. French professions usually have distinct masculine and feminine forms, and l'infirmier specifically refers to a male nurse.

Why is it à l' here instead of shrinking into au?

The word hôpital is masculine, but it starts with a mute 'h' which behaves like a vowel. Because of this vowel sound, we use the article l' instead of le. The preposition à only merges with le to make au, so à l' stays as two separate words.

Why does hôpital have a circumflex accent over the 'o'?

That little hat accent often marks a spot where the letter 's' used to exist in old French. Remembering this trick makes it easier to guess English cognates, like hôpital for hospital, forêt for forest, or hôtel for hostel.