Here's why:
* "After" is a conjunction. It connects clauses, and when it's used at the beginning of a sentence, it often feels like it's missing its other half.
* It can create a sense of awkwardness or incompleteness. The reader may wonder what the sentence is "after."
* It's often considered less sophisticated. While technically possible, it can sound informal or conversational.
However, there are some exceptions:
* Emphasis: You can start a sentence with "after" for emphasis, but it's important to do so sparingly. For example: "After all that hard work, we finally achieved our goal."
* Literary Style: In creative writing, authors may use sentence fragments and other non-standard grammar for stylistic effect.
Here are some ways to rephrase a sentence starting with "after":
* Use a different conjunction: "When the rain stopped, we went outside."
* Use a prepositional phrase: "Following the rain, we went outside."
* Invert the sentence: "We went outside after the rain stopped."
Ultimately, whether or not it's "proper" depends on the context and your intended audience. In formal writing, it's generally best to avoid starting a sentence with "after."