Sometimes when I put toothpaste on my toothbrush, I get somehow a drop of toothpaste in my eye, even though my toothbrush is more than 50 cm from my eye. How can I prevent this from happening?
You might want to consider using a lot less toothpaste. In contrast to what's commonly visualised in toothpaste commercials, a pea sized drop in the middle should be sufficient (see also here and here):
A Pea Sized Amount for Adults
For adults or anyone that’s old enough to brush their teeth independently (including children that can rinse well and floss on their own,) only a pea sized amount of toothpaste is necessary. Anything more than this is unnecessary.
Just depositing that pea in place removes the need to streak the toothpaste tube across the brush, which also completely stops the brush hairs from spattering.
You can also use something like this:
Might look silly but from what I can see, it would prevent any unwanted toothpaste in your eyes. :)