Are both OK?

Do you know what the best thing to do is?
Do you know what's the best thing to do?

Question

Are both OK?

Do you know what the best thing to do is?
Do you know what's the best thing to do?

Answer

Thank you for your question!

Only the first sentence is correct: “Do you know what the best thing to do is?”

The second one is not correct in standard English.

This is because when a question is inside another sentence (like after “Do you know…”), we do not change the word order. We keep it in normal statement order: the best thing to do is.

So even though a direct question would be “What is the best thing to do?”, in this case we say, “Do you know what the best thing to do is?”

Answer

Meaning: Same - asking for the best action.

Difference:
First sentence is formal, careful, grammatically “perfect”.
Second sentence is casual, spoken, friendly.

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