Which one is more natural? What's the differnece in meaning?

Question

● Which one is more natural?

● What's the differnece in meaning?

● Which sentence would you prefer to be used in an informal conversation?

1.You know what does cardiac mean, From what I can gather.

2.You know what does cardiac mean, as far as I can gather.

3.You know what cardiac means, I gather.

4.You know what cardiac means, this is what I believe to be true(=this is my guess).

5.In my point of view, you should have learned what (does) cardiac mean(s) till now.

● And my emphasis is on this question: What's the difference between "You know what cardiac means..." & "You know what does cardiac mean..."?

● Is one of them grammatically wrong?

Answer 1

In “you know what cardiac means” you expect that the person you are talking to knows what it (cardiac) means.

In “do you know what cardiac mean?” you are asking a question!

Just say:

“you know what cardiac means, I suppose.”

Answer 2

If you're not asking a question, then the phrasing "what does cardiac" is grammatically wrong.

What does cardiac mean?

Regarding the other part about your assumption; of your choices, the third one (I gather) would be most informal and natural. You can also use assume or presume instead of gather.

You know what cardiac means, I gather.

You know what cardiac means, I presume.

Assume and presume both mean "to take something for granted" or "to take something as true," but the words differ in the degree of confidence the person assuming or presuming has. Presume is used when someone is making an informed guess based on reasonable evidence. Assume is used when the guess is based on little or no evidence. - Merriam-Webster

Your fourth alternative, while not technically wrong, is the least natural and informal.

Your fifth alternative has a more accusatory quality where you seem to accuse someone for not yet knowing what cardiac means.