learning a new language is equal parts agony and delight

surgeon, technologist, writer, humanist. read.pourteaux.xyz 

What parts are the most agonizing and what parts are the most delightful to you?

some things don’t have any logic and just require memorization

for example, the difference between: 
十几个: a few more than ten
几十个: tens of 

the delight is the spark of understanding: even little things like comprehending a street sign or an overheard conversation

Isn't that just extending the same pattern of 十一，十二，十三... vs 二十，三十.... to 几, treating it as a sort of virtual number that follows similar rules to actual numbers?