What must sentences always begin with?

Sentences must always begin with a capital letter to indicate the start of a new thought or idea. This convention is a fundamental rule of grammar that helps readers identify the beginning of a sentence easily. Starting a sentence with a capital letter serves not only to signal the structure of the writing but also to convey the importance of each new statement. For example, in written English, "The cat is on the roof." clearly shows that "The" starts a new sentence, providing clarity and proper organizational flow.

The other options are not standard practices in sentence construction. A lowercase letter would not signify the beginning of a new sentence, making it confusing to the reader. A symbol, such as a comma or period, cannot logically start a sentence, as these punctuation marks serve different purposes in writing. Lastly, an exclamation mark is used to indicate strong feelings or commands and is not appropriate as the starting character of a sentence. Thus, having a capital letter at the start of a sentence is essential for following proper grammatical rules.