In Tetris, you are supposed to have a "bag" of five tetrominos that are drawn from in random order. You can see the tetromino falling down and which one is coming next, and you know the one after that must be one of the remaining three that haven't been drawn yet (assuming there are any). If all five have been drawn, then the bag is reshuffled. This mechanic is what constrains the randomness to something that you can reason about to design openers or other strategies, which is also why Tetris can theoretically be played forever.

As someone who grew up with Game Boy Tetris, this sounds much too easy, to the point of making it quite a different game. ;)

Correction, there are 7 tetrominos not 5. >.< Idk what I was thinking when I wrote that