I've only just recently begun learning the basics of time signatures and counting beats, and I'm having trouble applying what I've learned to real sheet music. Consider the second measure in this picture, a screenshot from Recondita Armonia, an aria from Tosca:

I believe the 3/4 time signature here means the song has three beats per measure and that the beats in question are quarter notes. But the second measure contains three quarter notes plus five sixteenth notes, which adds to more than three beats. So how is this consistent? What am I missing? Is it that the bars under the quarter notes change their value somehow?

