Choosing the right conjunction is important to make the meaning clear.
He is intelligent but hardworking.
He is intelligent and hardworking.
(Since the conjunction and connect similar ideas, it should be used in the above sentence.)
I have always been weak in mathematics and I have never failed a test.
I have always been weak in mathematics, but I have never failed a test.
(But is more appropriately used because the two sentences above have opposite ideas.)
When ‘and’ is used to connect two words or phrases in a sentence, no comma is used. Commas are used when there are three or more items in a sentence.
I had bacon and an egg.
I had bacon, a toast, and an egg.
We can start a sentence with a conjunction or place it in the middle of the sentence. A comma is used when a conjunction introducing a subordinate clause comes before the main clause as shown here. The subordinating conjunctions are in bold.
If he is not here soon, we shall leave without him..
We shall leave without him if he is not here soon.
When you see him, please talk to him about it.
Please talk to him about it when you see him.
Although she is my office colleague, we hardly talk to each other.
We hardly talk to each other although she is my colleague.
Because the weather is bad, we decide not to go out.
We decide not to go out because the weather is bad.
Before I go to bed, I have a glass of milk.
I have a glass of milk before I go to bed.
After the accident, he decided to give up his job..
He decided to give up his job after the accident.
(We use a comma when a conjunction introducing a subordinate clause comes before the main clause.)