你说的好what you said is good 你说得好 you speak well

So, they are not interchangeable in Chinese after the current rule was established. In your example, you should use 我写的练习 instead of 我写得练习. 得 is used in this structure: verb + 得 + adj/adv/clause etc 得 is used after the verb, and followed by an adjective, an adverb, a clause or other words and phrases which give additional information to the verb itself. e.g. 我写得很好，他写得很快，她写得很好看 but in your case, it is a noun look at this: 我的练习, and try to add more information to 练习 so as to make it more specific: 我写的练习（write）， 我画的练习（draw），我设想的练习（imagine），我打印的练习（print）,etc You will find in these examples, 写的，画的，设想的，打印的 are all used to provide more information to the noun （练习）， not the other way around.

There are many articles about the difference among 得，地 and 的. The previous answers are very good. I write this just to remind you that the current standard use of these three characters are recent manmade regulations in less than 100 years. You will find famous writers as late as in early Republic of China period who used them interchangeablely. The use of 得 and 地 is very clear. Remember when and how to use these two, and then use 的 in any other situations. My advice is to stick to the current standard rule but know early writings may not conform to this rule.

It would be helpful to remember that 得(dé) is preceded by a verb and followed by complement providing additional information about the verb. The complement could be an adjective, adverb or a clause. In contrast, 的 has completely different linguistic functions which are too many to be fully listed here, but 的 for sure has no such complementing function as that of 得.

Actually it is easy，you can remember like this first：得 with adj. others can ues 的.