How do I extract a single column from a data.frame as a data.frame?

Omit the ,:

x <- df[1]

   A
1 10
2 20
3 30

From the help page of ?"[":

Indexing by [ is similar to atomic vectors and selects a list of the specified element(s).

A data frame is a list. The columns are its elements.

Use drop=FALSE

> x <- df[,1, drop=FALSE]
> x
   A
1 10
2 20
3 30

From the documentation (see ?"[") you can find:

If drop=TRUE the result is coerced to the lowest possible dimension.

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