Dr. Naismith would have been surprised. If he had known how popular basketball would become.

What is the best way to combine the two sentences or clauses?

Answer: Dr. Naismith would have been surprised, if he had known how popular basketball would become.

Explanation:

The best way to combine these two sentences is to put a comma between them.

When joined together, these two sentences form a third conditional sentence.

In a third conditional sentence, one part of the sentence contains 'would have' + past participle (would have been surprised), while the other part of the sentence contains 'if' and past perfect form of the verb (if he had known). Third conditional sentences are used to talk about impossible outcomes. Dr. Naismith was a physical educator who died in 1939. Therefore, the scenario of him witnessing the popularity of basketball is not possible.