我写错了。我说对了。 these two sentences imply sth happened as we can tell from“了” at the end. When you want to mention the sth are already done you can use this kind of sentences. 我写得错。This sentence doesn't make sense. The structure of 得sentence is verb +得+adj /adv, just 错 can't be put after 得, but you can say 我错得很离谱，which means I did (sth) in a terribly wrong way. 我说得对。It's a right sentence. When you try to express how you do sth or by what methods, you are advised to use 得sentence .

Hi, the correct expression is "我写错了、我说对了”．＂我写得错、我说得对＂...it seems to use English logic to translate them. Sometimes, if you wanna give an opinion or agree with something, you may say '你说得很好，我赞成' “你写得不错＂ Hopefully it will help you.