CELSIUS: STAY CALM! is played primarily inside of a command line interface, but also features graphical elements.
At the top of the screen, the location indicator (eg. Your trusty, run-down cabin) shows you where in the game world you are.
The right hand side of the screen shows you the list of items Henri is carrying. Clicking an item in the inventory will give you every possible choice you can execute on it, a nice substitute to the "inv" command.
The center of the screen shows you all of the game text, including the items immediately apparent to you and immediately apparent directions Henri can walk in, as well as providing you a reference as to what's happening and what commands you previously executed.
The bottom of the screen houses the command line, which is where most actions will be done. Typing a command and then a destination item, location or person will execute it, assuming that command exists and can be run on that entity.
A few examples would be "go southeast", which will make Henri walk to the southeastern exit in a location if one exists and is unlocked, "look at mirror", which will make Henri look at at the mirror present in his inventory, "eat apple", which will make Henri eat an apple if he has one or one is in the room, and "use (item)", which will make Henri attempt to use an item. Other commands exist, and are usually easily discoverable by your own experimenting.
You can always use the 'inventory' tab on the right side of the screen to see all commands that can be used with an item you're carrying.