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One way to substitute a class in an HTML file based on a variable is to use JavaScript.

First, you need to create the variable and set its value based on your condition. For example:

var condition = true; // or false

Next, you need to use JavaScript to find the element in the HTML file and update its class. For example:

var element = document.getElementById("myElement");

if (condition) {
  element.classList.remove("oldClass");
  element.classList.add("newClass");
} else {
  element.classList.remove("newClass");
  element.classList.add("oldClass");
}

In this example, we first find the element with the ID of "myElement". Then, we check the value of the condition variable. If it is true, we remove the oldClass and add the newClass to the element's class list. If it is false, we do the opposite - remove the newClass and add the oldClass instead.

You can modify this code to use different conditionals and class names based on your specific needs.