OK, please let me answer yourquestion.
1."XX的" in Chinese is equal to a adjective in English. so when you translate an adjective into Chinese, maybe you should remember to take the "的". for example, in your sentence, "this new bag is beautiful", here, "new" and "beautiful" are both adjective, so it should be translated as "新的" or "漂亮的".
2.sometimes, when you can use Chinese flexibly, you will find some"的" can be omitted( and in Chinese, it means"省略"). And both"它是一件新衣服" and "它是一件新的衣服" are right.