Learn German: Die Krankenschwester hilft dem Arzt.

The verb helfen (to help) is a special verb in German that always requires its object to be in the dative case. Even though the doctor is receiving the help, we must use the dative masculine word for "the" (dem) instead of the accusative (den).

Why is the verb hilft spelled with an "i"? Isn't the infinitive helfen?

Yes, helfen is an irregular verb. For the "he/she/it" (er/sie/es) and informal "you" (du) forms, the stem vowel changes from e to i. This makes it sie hilft (she helps) and du hilfst (you help).

Since this lesson is about the imperative (command) form, how would I tell a friend, "Help the doctor!"?

To make a command for one person (the du form), you take the present tense du conjugation and drop the -st ending. Because du hilfst has that special e to i vowel change, it keeps the i in the command form. So, "Help the doctor!" becomes Hilf dem Arzt!

What does the word Krankenschwester literally mean?

It literally translates to "sick-sister" (from krank meaning "sick" and Schwester meaning "sister"). Historically, nurses in hospitals were often nuns, which is why the word for "sister" became part of the job title!