When you CAN use a comma (and no conjunction):
* The two clauses are short and closely related in meaning:
* Example: "The sun was shining, the birds were singing."
* The comma creates a clear pause and connection between the clauses:
* Example: "He looked tired, he had been working all day."
When you CANNOT use a comma (you need a conjunction):
* The clauses are long or complex:
* Example: "He decided to go to the beach, but he first had to pack his bags and find his swimsuit." (A comma alone wouldn't be clear enough)
* The clauses express contrasting ideas:
* Example: "She wanted to go to the party, however, she was too tired." (You need "however" to show the contrast)
* The clauses express a cause-and-effect relationship:
* Example: "He studied hard, therefore, he passed the exam." (You need "therefore" to show the reason)
Important note: The comma is always used with a coordinating conjunction (and, but, or, nor, for, so, yet) when connecting independent clauses.
It's generally better to err on the side of caution and use a conjunction, unless you're absolutely sure that the sentence is simple enough to be clear with just a comma.