I think there are some unintended consequence of the way ornamentation is notated in Western music. Often the ornaments are indicated with some kind of abstract symbol or with tiny “extra” notes (like grace notes), located visually outside of the music’s rhythmic structure. This sometimes leads less-experienced musicians to the conclusion, consciously or otherwise, that the ornaments do not have precise rhythms. Sometimes music teachers feed this problem by explaining the rhythmic aspects of ornamentation in a vague or misleading way.
For example, many of my saxophone and oboe students are initially stymied by this moment in the first of the Ferling 48 Famous Studies:
An unclear but common way to explain this is to indicate the pitch pattern of the turn—up a diatonic step and back down, down a diatonic (or maybe half-) step and back up—and then say something to the effect that these notes “steal” time from the F-natural. The grace notes in the next measure can be poorly explained by emphasizing that they go “on the beat.” These explanations aren’t factually incorrect, and make some sense to someone who already understands what the end result should sound like, but leave a lot of unanswered questions for students who are less experienced with ornamentation.
To be clearer about the turn, I think it helps to think through exactly how many notes have to be played in the space of the F-natural (five) and some possible ways to fit them in. Here are a few:
Since there are multiple possible rhythms that can accommodate the notes involved, there is perhaps a bit of artistic decision-making that needs to take place, but this does not mean that the execution of the ornament is left to chance. Pick one (the third one sounds best to my ear—it gives the initial note of the turn a little extra weight), and practice it carefully and precisely with the metronome, just like you would practice any other rhythmic figure. It wouldn’t be a bad idea to write the rhythm into your score, so you don’t panic and forget the rhythm under pressure.
For the grace-note figure, it’s not terribly difficult to simply put the notes “on the beat,” but don’t leave to chance the placement of the non-grace G. Select a precise duration for the grace notes so that the G happens on purpose, in a place that makes rhythmic sense. I like this:
The etude also includes several trills. It’s tempting just to think that all you have to do is waggle a finger for a while, but there are some nuances that are difficult to execute well if the precise rhythm of the trill is left to chance. For example, in the following passage I like the trills to have a subtle acceleration, and to end on the lower note so that I can move smoothly into the grace notes (which, in this case, I would place before the beat).
If I’m just waggling, it’s very easy to let the rhythmic accuracy suffer. Here’s one possible solution that sounds good to me; notice that I’ve built the acceleration right into the rhythm so that I can practice it really slowly and carefully with my metronome.
Make no mistake: the notes of an ornament are real notes, with pitch and timing and duration and style, just like the notes that are spelled out explicitly on the page. The symbols are merely a shorthand for writing the precise rhythms. To make sure they turn out exactly right every time, it’s up to you to expand them into real rhythms and practice them accordingly.