There are 6 buttons and a text interface on the top. The button that says "0" we can ignore. If something doesn't work press the button try it again and tell me what it says. This is the command block output. This will also appear in chat. It will spam your chat so I would recommend typing /gamerule commandBlockOutput false. Like with give you can press tab to autocomplete after typing "com". Don't forget the last bit aswell. The done and cancel buttons (can also be activated by enter and escape respectively) are what you use to get outside the interface. Cancel will not save what you did since you last opened the command block. Done will save what you did. That leaves the button saying impulse and the button saying needs redstone. The button saying impulse will switch between the mode of your command block. Impulse, chain and repeating. For now just think of impulse as what is your chat. That leaves us with needs redstone. That button can be either needs redstone or always active. Needs redstone means it will power at when redstone is powering it. Always active means it is always active. For impulse always active is quite useless as it only activates once and that is when you first press done after making it always active.

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Now onto chain and repeat. And we will talk about the middle button aswell. It goes with chain. First repeat. Repeat will run 20 times a second as long as its powered either by redstone if its on needs redstone or if it's always active. Now onto chain. This is a bit complicated. But basically it will run in the same game tick (1/20 of a second) as the starting command block. It has to start with an impulse or a repeat. I recommend always setting chain command blocks to always active. Then chain command blocks will only power if there is a command block pointing into them. You can tell by the arrows.