Do you have to pay tax if you marry a U.S citizen?

The marriage penalty in the United States refers to the higher taxes required from some married couples with both partners earning income that would not be required by two otherwise identical single people with exactly the same incomes. Multiple factors are involved, but in general, in the current U.S. system, single-income married couples usually benefit from filing as a married couple because of income splitting, while dual-income married couples are often penalized in comparison. The percentage of couples affected has varied over the years, depending on shifts in tax rates.

Only if you didn't pay taxes in your own country. Most countries in the world have arrangements when it come to that. Most will accept proof of payment.

I would recommend keeping your accounts separate because if you put money in an account that belongs to your spouse, that could be considered income. It's a tough one. Talk to an accountant here and there.

If you live in the US then yes, you have to pay taxes whether or not you are married. If you live outside the US, then you only have to pay US taxes if your spouse files as married filing jointly. If they file as married filing singly then your income etc. are no business of the US government.

If you live in the United States, you have to pay taxes.

Yes

of course you pay tax on any earniings