Alternative ways to enter English letters in Ubuntu

Try the old technique of using Alt-codes.

Hold down Alt, then type in the two or three digit number that corresponds to the desired letter, then release Alt.

Make sure to type the number on the numpad. (Yes, this might require holding Fn if using a laptop keyboard with a reduced size.)

To see what number to type, check an ASCII table like what is seen at http://www.asciitable.com/

in short, 65 is capital A, 66 is capital B, 67 is capital C, and so on. Lowercase letters are 32 higher, so lowercase a is 97, lowercase b is 98, and so on.

13 typically represents the same thing as pressing the Enter key.