Questions & Answers about Вчера девочка читала письмо и улыбалась.
Why does улыбалась end in -сь instead of the usual -ся?
Since улыбалась is a reflexive verb, does it mean she was smiling at herself?
No, she is just smiling! While the -ся or -сь ending often means doing an action to yourself, some Russian verbs are just built with this ending and do not carry a self-directed meaning. Улыбаться (to smile) is one of these words—it is always reflexive.
I thought the object of a verb needs to change its ending. Why is it just письмо here?
It actually is in the accusative case (the case for direct objects), but the change is invisible here. Письмо is a neuter noun. In Russian, neuter nouns and non-living masculine nouns look exactly the same in the accusative case as they do in their dictionary form.
Notice how both читала and улыбалась have an -ла in them. Is that required?