Hi! Sorry for my english. I got a question, why when I choose 7th or 9th etc chords or Inversions of chords I see incorrect note position in chords. Example, 9th chord have to consist of 5 sounds, but it shows only 4, triads have to be three notes, but there is 4. I don't even understand how inversions working there, it's just random. Not as I learned from Hooktheory
Hookpad displays all the notes with a chord. So a C major chord would show all octaves of C, E, and G. Inversions are included in these octaves, so it's not shown visually, but you can hear the difference. This explains the "4" notes that you see in the triad. It's just one of the notes up/down an octave.
In Hookpad, there is a maximum limit of 4 notes that can be in a chord. Therefore, you can't have the 5-note 9 chord you're talking about. After some research, the "9" chord that Hookpad has doesn't have the 5th (so 1-3-7-9), while the add9 chord is a 7-9 chord without the 7 (1-3-5-9).
Also, Hookpad has the limitation of only 7 chords having inversions. You can't have a I64sus4 chord for example. Inversions and embellishments in Hookpad are mutually exclusive.