Learn Spanish (Latin America): Debe de ser el cartero porque el perro ladra. - It must be the mail carrier because the dog is barking.

The phrase deber de plus an infinitive is used to make a deduction or assumption, meaning "must be" (as in "I assume it is"). If you leave out the de and just say debe ser, it traditionally implies an obligation ("it is required to be"). Note that in casual, everyday speech, many native speakers will just use deber for both!

Where is the word for "it" in debe de ser?

In Spanish, the subject pronoun "it" is almost always left out. The "it" is already built into the third-person singular verb debe.

Since the dog is barking right now, why does the sentence use ladra instead of está ladrando?

In English, we usually use the present continuous ("is barking") for actions happening at this exact moment. In Spanish, the simple present (ladra) can mean both "barks" and "is barking." While you absolutely could say está ladrando here, using the simple present for right-now actions is completely natural and very common.

If the mail carrier were a woman, would I say la cartera?

Yes, you would! Just rely on the context of the sentence so people know you mean a mail carrier, because la cartera is also the standard word for "purse" or "wallet" in many Latin American countries.