When he have independent clauses (sentences that can stand on their own), and want to join them, we can use a comma and either a coordinating or a subordinating conjunction.
First let’s deal with coordinating conjunctions:
Known as FANBOYS, these seven conjunctions will help you remember the coordinating conjunctions. It is important that you know the difference between these conjunctions and that you use the conjunctions appropriately.
Ex. 1) Kelly wanted to get straight A’s, and she studied every night and weekend.
Ex. 2) Kelly wanted to get straight A’s, so she studied every night and weekend.
In the first instance, we have ‘and’ and in the second we have ‘so.’ Only one of these words is correct. The other is an example of faulty coordination.
Kelly wants to achieve something, SO she does such-and-such. Therefore, the correct version of the sentence is Ex. 1).
Now let’s take a look at the following sentence:
Ex.1) Kelly was able to score at the top of the class, and she was even considered for the position of valedictorian later on.
Ex. 2) She was able to score at the top of the class, but she was even considered for the position of valedictorian later on.
‘But’ and ‘yet’ are conjunctions that show contrast. ‘And’ is a conjunction that shows that two independent clauses are related. In the examples directly above, we want a conjunction to connect, not to contrast, the idea that Kelly did two important things. So the correct answer is Ex. 1).