As my students get better at reading more complicated rhythms, often it is the “easy” notes that emerge as the ones still lacking in precision: the longer note values and the rests. It’s not uncommon for my university students to play intricate sixteenth-note rhythms with accuracy and confidence, but play whole notes that are too short by a significant fraction.
Try this: use your thumb and index finger to indicate a distance of about an inch (or, if you live in a country with saner units of measure, do a centimeter). How sure are you of your estimate? Now try one foot (or maybe 30 centimeters), then one yard (or one meter). How sure are you of those estimates? Probably less so. Look into the distance and see if you can guess at a point that is a mile (or kilometer) away. At distances this large, it starts to feel more like just guessing. But if you are reasonably confident of your smaller measurements, you can use them to derive the larger ones: measure out 12 of your guesstimated inches, and you can be more confident that you are in the ballpark of a foot.
Note values are the same way: it’s much easier to place notes that have less “distance” (time) between them. This is why subdividing is key to playing longer notes (or rests) accurately. Don’t start a whole note, wait, and end the note when you suppose enough time has passed; instead, mentally use smaller notes to measure out the longer ones. For example:
Subdivision is also particularly useful for slowing down tempos smoothly and gracefully. When the “large” beats need to get farther apart, it’s not easy to do this gradually and evenly. Smaller beat subdivisions are much easier to manipulate.
Subdivision takes some concentration at first, but can become somewhat automatic with practice. Master this technique for greater precision and control of tempo.