Here's why:
* Flow and emphasis: Starting a sentence with a preposition like "at" can disrupt the natural flow of the sentence and make the subject seem less important.
* Clarity: While not always the case, starting with a preposition can sometimes make the sentence harder to understand, especially if it's a long or complex sentence.
Examples:
* Less formal: "At the park, I saw a squirrel."
* More formal: "I saw a squirrel at the park."
When it's okay:
* Short, informal sentences: Starting with "at" can be fine in casual writing or speech, especially if the sentence is short and simple.
* Emphasis: You can use "at" to emphasize a specific location or time. For example, "At this very moment, I'm feeling excited."
Alternatives:
* Consider using a different preposition: "In the park, I saw a squirrel."
* Reword the sentence: "I saw a squirrel while I was at the park."
Overall, there's no hard and fast rule against starting a sentence with "at." But it's often a good idea to consider the context and choose an alternative if it makes the sentence sound more natural and flowing.