Old style ASCII input without Ctrl

This is an example from an old book: "The Mma book 5th ed". It was called "Entering 2D input". If I paste the following in a notebook:

\!$$\[Sum] \+ \( n = 0$$ \% m 1 \/ f[n] \)


it promptly gives me a formatted expression but I can't enter the same text in an input cell on my own. It is curiosity more than anything but could someone please point out what I am doing wrong?

• Esc-sum-esc gives the $\Sigma$. "sum" is 3 lowercase letters. Ctrl-underscore puts you in subscript mode. n=0 is the index initializer. Ctrl=% goes from subscript to superscript. 5 is the upper limit. Ctrl-spacebar goes from superscript to plain input. 1/f[n] is the summand, followed by shift-enter to evaluate the expression. Oct 30 '21 at 10:24
• @LouisB : I know the shortcuts with CTRL. I can't enter this legacy expression (shown with an error above) in an input cell, although I can paste it as seen in In[1]. The notebook cell is okay with the paste but somehow does not accept it character by character. I was wondering why. Thanks for your kind response.
– Syed
Oct 30 '21 at 10:50

According to my test, the functionality is introduced in v3, but auto conversion doesn't happen in any version if one directly types the expression inside the notebook. In v3 and v4, one needs to manually convert the cell to StandardForm by e.g. pressing Ctrl+Shift+n as the last step:
In v5, one more extra step is required. One needs to first create a cell and convert the cell to InputForm by e.g. Ctrl+Shift+i, then enter the code and convert the cell back to StandardForm.
Start from v6, raw InputForm is introduced, and one needs to create a cell and convert the cell to raw InputForm by e.g. Ctrl+Shift+r, then enter the code and convert the cell back to StandardForm. This remains to be the behavior of v12: