Enough theory to read the scoring summary and know what to fix. Subdivisions, the grid, swing ratios, ghost notes, and what "the pocket" actually means when a computer grades it.
A bar of 4/4 is four quarter notes. Cut each in half and you get eighths. Cut those in half and you get sixteenths. Finger drumming lives mostly in eighth-note hat patterns with sixteenth-note ornaments. Count out loud ("one and two and three and four and") until you stop having to.
When Padforms grades a hit, it compares your timestamp to the nearest gridline. Perfectly on the gridline is a perfect hit. A few milliseconds early is "early"; a few late is "late". Most musical feel sits in a window of plus-or-minus 25ms; outside that window you start to hear it as wrong.
Straight eighths split each beat into two equal halves. Swung eighths push the second half later, so the pair sits as "long-short" instead of even. The ratio decides how much: 60/40 is light shuffle, 67/33 is hard swing, anywhere past 75/25 starts to sound like triplets. Hip-hop tends to live around 54/46, where the swing is felt more than heard.
Ghost notes are deliberately soft hits, usually on the snare, that fill the spaces between the loud snare backbeats. They're what makes a beat sound like a drummer rather than a drum machine. Velocity matters: a ghost at velocity 30 reads differently from one at 60.
The pocket is the small window where every instrument lands within a tight range around the grid, consistently, bar after bar. Bigger than that window, you hear rushing or dragging. Smaller, you hear stiffness. The pocket is wide enough to feel human and narrow enough to feel deliberate.
Most beats are in 4/4 because four-beat bars line up with how people walk and dance. But 6/8 (six eighth-notes per bar, felt as two groups of three) gives you ballads and shuffles. 5/4 and 7/8 are odd metres that show up in jazz, fusion, and a lot of modern beat music. The counting changes; the placement logic doesn't.