This code does not work!!!

[amindanial]

The following code does not work, whereas it works fine on Mathematica.

[JerryI]

I did not know if you applied Set to a pattern on purpose, but with a regular symbol it works

PS: If it is a way of naming a variable, I would not recommend to use snake case at all, since underscore is a reserved syntax for Blank and the fundament of pattern matching of the whole WL. And it will also create unnecessary overloads on blank symbol and your symbol e.

[JerryI]

Applying D to a such pattern might lead to an undefined behavior.

[JerryI]

experts came - @KirillBelovTest ;)

In your first line you make a definition, that any symbol with a head x is d[t] Cos[...]
that means, if you just evaluate e_x it returns an empty pattern, however, x[] or x[1] will return you d[t] Cos[...]

So again, don't use snake case for your variable names. There is no variables in WL, only symbols and they must be either in pascal case or camel case, otherwise it is no longer a definition of a plain symbol, but more complicated pattern definition.

[amindanial]

Thanks a lot.

[amindanial]

How can I write a variable as shown in the following figure:

These commands were imported from a Mathematica file and fortunately kept the format.

[JerryI]

Easy.

Option 1: Select your symbol and hit Ctrl+_ like in Mathematica, it will create a subscript

Option 2: Open command palette and call Insert Special Characters. It will create a panel with most math decorations. Again, select your symbol and click on the desired icon.

PS: If you copy and paste the text from the editor to a notepad you can see, that it is just

Subscript[e, x]

[JerryI]

Here you can find all shortcuts

https://jerryi.github.io/wljs-docs/frontend/Cell%20types/Input%20cell

[amindanial]

Many Many Thanks