Question about Texas Hold Em

I was trying to play Texas Hold Em poker with some friends recently, and we had a question come up in play that we were not sure about.

The person who was to put in the Big Blind (supposed to be $800), did not have enough money to put in the full Big Blind, so they went all-in with the $200 that they had.

The Small Blind was $400 (half the big blind).

The question was, does the next player after the Big Blind have to only match whatever the highest bet on the table is (ie. the $400) or do they have to bet the already set amount for the Big Blind of $800 to stay in the game?

Thanks

They have to call for $800, the $600 excess going into a sidepot.

Everyone has to put in $800. If the small blind was the only one to see the flop with the all in guy, he would recieve $200 of his Small blind back and they would gamble for the $200 in the middle. If one person(NOT the small blind) called, he would recieve 600 of his chips back, and 200 of the Sb. If more than one person called, for every player 200 would go in the main pot for the shortstack to win and the rest would go in a side pot, where players can keep betting. Most of the time unless the sidepot players have a big hand they just check it down to try to knock the shortstack out.