They each sound natural enough, but neither one is a sentence. They're both dependent clauses. If you eliminate the word "because" from the beginning of each one, they would be independent clauses that could stand on their own as complete sentences.

Sentence 1: "Because he knew that he would stand to gain a lot just by hanging out with you." Error: Fragment: This sentence is a dependent clause, meaning it doesn't stand on its own as a complete thought. It starts with "Because," making the reader expect a reason or consequence, but it's not followed by an independent clause. Correction: Add an Independent Clause: You can fix this by adding what happened as a result. For example: "He called you every day because he knew that he would stand to gain a lot just by hanging out with you." Sentence 2: "Because he knew that just spending time with you would be advantageous for him." Error: Fragment: Like the first sentence, this one is also a dependent clause and does not complete the thought. Correction: Add an Independent Clause: You can correct it in a similar way to the first sentence. For example: "He made plans with you because he knew that just spending time with you would be advantageous for him." In summary, while the internal structure and grammar of both sentences are correct, they lack complete thoughts and are thus sentence fragments. Adding an independent clause that explains the consequence or action related to the reason stated in these sentences would make them complete and natural-sounding.

Both sentences convey the same idea and are grammatically correct, but they have slightly different tones and levels of formality. Here's a breakdown of each sentence: "Because he knew that he would stand to gain a lot just by hanging out with you." This sentence sounds natural and conversational. The use of "just by hanging out with you" emphasizes the simplicity of the action (hanging out) leading to a significant outcome (gaining a lot). It has a slightly informal tone. "Because he knew that just spending time with you would be advantageous for him." This sentence is more formal and uses the word "advantageous" to convey the idea of gaining something beneficial. It sounds a bit more structured and could be suitable for a more professional or formal context. Both sentences are correct and natural, but the choice between them depends on the context and the level of formality you want to convey.

I agree with JK and Marie that they are not sentences. However, you can turn them into sentences by placing a comma after "because". That way, "because" becomes isolated and could possibly be treated not as part of the phrase, but as a side-thought like "alas!" or "well!" or "so". The real sentence would then be "He knew that ...". However, even with the comma, the sentence would lack needed context. Additional sentences would be needed to make clear what in the world you are talking about.

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