Yes, all those single-letter names are just ordinary variables that you can overwrite. Doing so is nearly always a terrible idea.

The article is being a little unclear when it says "cannot use". You can use literally any variable name in R if you really want to. If the name you want is already a reserved word (e.g. "for", "else", "function"), or if it is not a syntactically valid token (e.g. '@!":%$>"@;'), then you just have to enclose it in backquotes. So the following is valid R:

    `for` <- 1:5
    `function` <- 5:1
    `TRUE` <- `for` / `function`
    `@!":%$>"@;` <- `TRUE`^2
    print(`@!":%$>"@;`)

and if you overwrite variables like "c", you can always invoke the original concatenation function as "base::c".

You don't have to declare a variable before you use it, so there's no such thing as "a new T".

But can I declare the local t in the function? And is the t in the function parameter local?

Yes and yes. R is lexically scoped

It's not really as bad as all that. The reason to avoid using those names is to avoid confusing the reader, not because it causes problems for R

You can cause some mischief - if you write a package, or distribute a .Rprofile to colleagues, you can hide somewhere in there a

temp <- F

F <- T

T <- temp

and if your colleagues are lazy and use T and F (instead of TRUE and FALSE) fun and very gnarly to debug things are going to happen!

Nichts ist wahr! Alles ist erlaubt!

There are way worse things you can do than that! e.g.

`(` <- function(x) { if (is.numeric(x) && runif(1) < 0.1) { x * 1.1 } else { x } }

;)

That is beautiful, thank you very much! That's the kind of "bug" people don't even notice.