A. NO CHANGE B. viruses; however, C. viruses, however, D. viruses, however,

Understanding grammar rules is essential to mastering the art of writing and speaking in English. One of the foundational grammar rules involves the appropriate use of punctuation to clarify the meaning of sentences. Punctuation marks, like commas, periods, semicolons, and question marks, indicate pauses, separation of ideas, or the end of sentences.

In the case of the exercise, punctuation's role is to separate independent clauses within a sentence. An independent clause is a group of words that can stand alone as a sentence, expressing a complete thought. A common mistake is to join these clauses incorrectly, leading to run-on sentences or comma splices.

For example, when you see the word 'however' in a sentence, it usually indicates a contrast or turn in the sentence. It acts as a signpost to the reader that the direction of the sentence is about to change. The appropriate way to punctuate around 'however' depends on its function and where it is in the sentence. A semicolon before 'however' and a comma after it signify a pause that distinguishes two distinct but related thoughts.

Remember, each punctuation mark has its own set of rules, and using them correctly can greatly improve the clarity and professionalism of your writing.

Proper sentence structure is another critical element of English grammar. When constructing sentences, the goal is to communicate an idea with clarity and coherence. Each sentence must have at least a subject and a verb to be considered complete. Beyond these basic elements, sentence structure can become more complex, incorporating multiple clauses and varied sentence types.

Subjects, Verbs, and Objects

At its simplest, a sentence contains a subject (who or what the sentence is about), a verb (what the subject is doing), and often an object (who or what the action is done to). Complex sentences may have additional clauses, both dependent and independent.

Combining Clauses

Independent clauses can stand alone as sentences, but sometimes combining them provides more nuanced or efficient communication. To combine independent clauses correctly, you should use a comma and a coordinating conjunction or a semicolon.

Incorrectly combining clauses can result in grammatical errors such as run-on sentences. It is also crucial to ensure that each clause has a clear subject and a verb so the reader can follow the sentence's logic.

Conjunctive adverbs are words that connect one clause to another within a sentence. They serve to create a smooth transition between ideas, often indicating a contrast, sequence, cause and effect, or other relationships between the clauses. Common conjunctive adverbs include 'however', 'therefore', 'moreover', 'nevertheless', and 'consequently'.

Punctuating conjunctive adverbs can be tricky for many students. Let's take 'however' as an example from the exercise. 'However' is often used to introduce a contradiction or an opposing view. It must be preceded by a semicolon or period when connecting two independent clauses and followed by a comma.

Examples:

• The team played well; however, they lost the game.
• I wanted to purchase the car. However, it was too expensive.

As these examples show, conjunctive adverbs are more than just fancy transitions; when used correctly, they add sophistication to your writing by highlighting intricate logical connections between ideas.