I have heard this situation explained this way: When you want to connect two independent clauses, which can stand alone as sentences, then you can connect them with a Coordinating Conjunction, also called a FANBOYS word. (FANBOYS is an acronym reminding us of the seven coordinating conjunctions, which are For, And, Nor, But, Or, Yet, and So.) Normally, these FANBOYS words should indicate the connection between the two independent clauses. Here's an example: 1. I usually drink orange juice with breakfast. (Independent clause, and Simple Sentence.) 2. I prefer to drink cold milk when I eat pancakes. (Also an independent clause that can stand alone as a sentence.) Now, to combine these two simple sentences into a Compound Sentence, we use a comma followed by a FANBOYS word. Since these two sentences are contrasting the drink preference of a normal breakfast with that of pancakes, it would be best to use the coordinating conjunction BUT to connect these independent clauses like this: "I usually drink orange juice with breakfast, but I prefer to drink cold milk when I eat pancakes." Notice that the period of the first sentence is replaced with a comma, the conjunction "but" follows, and then the second independent clause finishes the compound sentence. If the second sentence/independent clause begins with a word that wouldn't normally be capitalized, then you must make it lowercase. Here's an example: 1. Teachers encourage students to read daily. 2. Students choose video games over reading more often than not. Because these two independent clauses are making statements that should have a connection, but don't, we can use "YET." And again, the word "students" is not capitalized. "Teachers encourage students to read daily, yet students choose video games over reading more often than not." I hope this helps and that it's not too complicated. (I'm no expert!) An English grammar handbook with a writing chapter could help also. PEACE!

The rule is generally presented like this: S V, coordinator S V. This means two complete sentences connected by short coordinators like "and, or, nor, but , so" will require a comma between them. "Two complete sentences" means Subject and Verb. For example: I like cake, but my brother likes brownies. (S V, but S V) If there is no second subject, you do not have a complete sentence. For example: I like cake but don't like cookies. (S V but V)

'He likes dogs' is an independent clause. 'and he wants to keep one' is not an independent clause because 'one' refers to dogs. Remove the 'he' from the sentence since we already know that you are talking about him. He likes dogs and wants to keep one. 'He likes dogs' is an independent clause. 'he wants to keep the Rottweiler' is an independent clause. He likes dogs, and wants to keep the Rottweiler. Why does your brother have a puppy in his hands? He likes dogs and wants to keep one. Why does your brother have a puppy in his hands? He likes dogs, and wants to keep the Rottweiler.