Which statement is correct regarding sentence structure?

The statement about a sentence always beginning with a capital letter is indeed correct. In English grammar, the convention is that the first word of every sentence should be capitalized. This rule helps to signal the beginning of a new thought and aids readability. While there may be exceptions in creative writing or poetry, the fundamental rule in standard written English dictates that sentences should start with capital letters to ensure clarity and proper structure.

Other options may present valid points, but they do not hold the same foundational significance in defining sentence structure as the capitalization rule does. For example, while it is true that many people may forget to use commas correctly, this does not pertain strictly to what constitutes a sentence. Similarly, the requirement for a subject in a sentence is a general rule; however, there are instances in informal contexts where sentences may appear to lack a clear subject. Starting a sentence with a conjunction is also possible in literary and informal contexts, but it tends to be a stylistic choice rather than a grammatical necessity. Therefore, the rule about capitalizing the first letter of a sentence remains vital in establishing a standard for sentence structure.