Learn Russian: Она мило улыбнулась и ушла.

It tells us they happened in a completed sequence. Using consecutive perfective verbs in the past tense means one action finished completely before the next one started: first she smiled, then she left. If the sentence used the imperfective улыбалась, it would imply she was smiling while she was leaving.

How does the prefix у- change the base verb of motion in ушла?

The base past-tense verb шла simply means "was walking" or "was going" (in one direction). Adding the prefix у- creates the perfective verb уйти (past tense ушёл / ушла), which specifically means "to leave" or "to go away" from a location on foot.

Why does the reflexive verb улыбнулась end in -сь instead of the standard -ся?

Reflexive endings change based on the letter right before them to make pronunciation smoother. Because the feminine past tense ends in a vowel (-а), we attach the shortened -сь. If it were a masculine subject, the verb would end in a consonant and take the full suffix: он улыбнулся.

This lesson focuses on the exclamatory phrase что за. If you wanted to react to her action by exclaiming "What a sweet smile!", how would you say it?

You would say, "Что за милая улыбка!" The phrase что за is used to express surprise, admiration, or sometimes indignation. It acts as an identifier and is always followed directly by the noun phrase in the Nominative case.