In the following sentence:
学习汉语的人一般都会认为汉字“难认”和“难学”,其实,这主要是在开始学习的时候。随着学习的深入,难度会降低,“难学”甚至会转化为“易学”,这是因为汉字在音、形、义三个要素之间有着不少内在的联系。
深入 is a verb, and 学习 is also a verb or noun. So it is awkward 学习 modifies a verb here.
One possibility is that 的 is misspelled with 得 here and 深入 is used an an adjective (meaning "thorough" or "deepgoing"), but this text is formal and not something you see on the web (it is a textbook for Chinese learners).
So why is 的 used in such a way? What does it mean here, and is it grammatically correct? Or is it simply a typo with 得 (which sometimes happen among native Chinese speakers, though as I said, it is a textbook for language learners and should be checked before pubilshed)?