Learn Dutch: Er zijn heel weinig goede cijfers. - There are very few good grades.

Weinig has irregular comparison forms, which is exactly what this lesson covers! To say "fewer", you use minder. For "the fewest", you use het minst. So, "fewer good grades" would be minder goede cijfers.

Why does goede get an -e ending, but weinig doesn't?

Goede gets an -e because it is a regular adjective modifying the plural noun cijfers. However, weinig (few/little) acts as a word of quantity here. Just like veel (much/many) and specific numbers, it usually doesn't get an -e ending when placed before a noun.

Does cijfers mean "grades" or "numbers"?

It can mean both! The literal translation of cijfers is "numbers" or "digits" (singular: cijfer). However, in a school or testing context, it is the standard Dutch word for "grades" or "marks".

Could I use erg instead of heel to say "very"?

Yes, erg weinig and heel weinig both mean "very few" and both are perfectly correct. Heel weinig is just a very common and natural-sounding combination in Dutch.