[Please ignore my other answer. I didn't do it right.] I always drink coffee with milk. I never drink it black.

The sentence "I always drink coffee with milk, I never drink it black" is a run-on sentence as it contains two independent clauses separated by a comma. You can correct it by either separating the two clauses into two separate sentences or by using a conjunction to join the clauses. Here are two possible corrections: I always drink coffee with milk. I never drink it black. I always drink coffee with milk, but I never drink it black. In the first correction, the two clauses are separated into two sentences, which makes the sentence clearer and easier to understand. In the second correction, the coordinating conjunction "but" is used to join the two clauses, which also makes the sentence clearer and more concise. Overall, it's important to avoid run-on sentences in your writing, as they can make your meaning unclear and confuse your readers. By using proper punctuation and conjunctions, you can ensure that your sentences are clear, concise, and easy to understand.

I agree with our learned friend Ahmadi, of course, except in one thing. I don't think using "but" as a conjunction is advisable here. The second clause is not in any way contrary to the thought or idea of the first. It is rather a reinforcement. In such cases, you might consider using "and". In this case I suggest using no conjunction. Just start a new sentence. What you have written is a comma splice just as the other answer said.