得 is always used in oral Chinese, it means you need do something or you should do something. While 一定 can be used into both oral Chinese and articles, it means you must do something.

得（de2）is commonly used in speaking escecially those northener in China.

得 sometimes means must,seldom used in sentence. 一定 means certain 某些, 一定, 某, 肯定, 确定, 必然 given 特定, 一定, 赠予的, 斯 fixed 固定, 定, 一定, 确定, 既定 particular 特别, 特定, 特殊, 具体, 特, 一定 certainly 当然, 一定, 无疑, 的确, 毫无疑问, 必 necessarily 一定, 必 surely 一定, 定, 保管, 的确地 for sure 一定, 必然, 准保

The potential complement de(得) is used to show the outcome of an action or event. This action can be habitual (i.e. he walks slowly) or a particular event (i.e. he sang well last night). The potential complement de is placed after a verb. This structure is followed by a phrase which shows the result of the preceding action: VERB + de 得 + RESULT 他說得很好。(Tā shuō de hěn hǎo) He speaks well. 香港比北京的天气热得多。(Xiāng Gǎng bǐ Běijīng de tiānqì rè dé duō.) The weather in Hong Kong is much hotter than Beijing. 一定（Yīdìng）used for both adjective and adverb: For adjective: certain, given,fixed, particular Ex-只要你努力一定会成功 For adverb: certainly，necessarily, surely,for sure Ex-我一定會去參加你的婚禮｡

we don't use ’得‘ in written English but we can use 一定 in both written English and common speak