How to play a triplet and two eighth notes at the same time.
From Chopin’s Prelude in E Minor (op. 28, no. 4):
The difficulty is this rhythm:
Triple the note lengths to obtain a more familiar rhythm.
An eighth note becomes a dotted quarter note:
A triplet’s eighth note becomes a quarter note:
So our rhythm becomes:
This looks more familiar and makes us realize that, in the original rhythm, the second eighth note falls exactly in the middle of the last two triplet notes. So we are now able to play accurately the rhythm at a slow speed.
At a normal speed, we need to think of the whole rhythm as one beat. We can use the ternary rhythm:
With the two hands:
For example, this bar:
can be thought of as:
Try it. It looks complicated but it's quite easy.
As you get more comfortable , try to hear the two rhythms (triplet versus eight notes) separately while playing. It will make them sound more independent, as they are supposed to.
I use the metronome app Tempo Advance to quickly hear how a polyrhythm sounds.