What does "si" mean in "Lui aspetta" vs. "Lui si aspetta"

Question

What significance does si have in the two sentences?

• Lui aspetta
• Lui si aspetta

I understand si means himself/herself/themselves(singular).

I translate the two phrases naively as "he waits", and "he himself waits". (Or he expects vs. he himself expects) But Google Translate and Duolingo tell me the differences are actually:

• He waits and,
• He expects, respectively.

So, what language rule am I missing here?

Grazie in anticipo!

Answer 1

The form aspettarsi is formally reflexive, but it's not the same reflexive as lavarsi (to wash oneself): it's impossible for anybody to “wait for themselves”.

It's actually a “medial” form, which denotes “advantage”. A common case is

mangio una mela

mi mangio una mela

The latter form is sometimes mistakenly considered ungrammatical, but instead conveys the idea that I'll eat an apple and will be happy to.

Different verbs have different meaning of the medial form. The verb aspettare means essentially the same as to wait for; the medial form indeed means to expect in almost all meanings of English. An exception: in English one says to expect a baby, in Italian it is aspettare un bambino (not *​aspettarsi). Another exception: to be expected is rendered as essere atteso; in the same vein the expected value (in probability theory) is valore atteso.

The verb attendere/attendersi is pretty much a synonym for aspettare/aspettarsi.