得 is only used for complements. It shows up usually after verbs, sometimes after adjectives. But the thing after it is always always always some kind of complement. If you're not sure what chinese complements are, I'd do some internet research ir review your textbook, they'll explain better than me. The sentence you posted is basically just noun phrase a(我学的)=noun b(中文), right? There is no verb complement. And no adverb, so it can't be 地。 so by process of elimination, if nothing else, it's gotta be 的。 我学的 is basically an example of the closest chinese to a relative clause. It's used to form complex or highly specific adjectival phrases, which, when not modifying a noun, can act as nouns. It maybe sounds complicated but it really isn't. Take a look at the example sentences below (我妈昨天给我做的蛋糕)特别好吃。 (The cake my mom made for me yesterday) was really delicious. (住在中国的外国人)不都会说中文。 (Foreigners who live in China) dont all speak chinese. 我的狗是(世界上最可爱的)。 My dog is (the cutest in the world). Do you see how de is being used these three sentences? By putting it after a phrase you turn it into a noun or adjective. Hope that clears things up a little. lemme know if you still need help or have any other questions.

Glad I could help! It’s also worth noting that in casual setting people often just use 的 for all three. Your teacher will get mad if you do this and it doesn’t look good in a formal essay or email. But native speakers do it, especially in texts. Don’t get confused by it.

It depends on the word in front. V+得 , the others + 的 You can generally apply this principle. And If you are really confused about what to choose, just choose 的. It's not a big deal and we native don't really care about it actually.

得 后 接 形容词 例如：我的中文学得很好

我学的(东西)是中文 = 我学的是中文