"the letter xee"
Translation:το γράμμα ξ
Yes, because that is the name of the letter so it is singular. Thank you for your very interesting question.
The best way, which is also very easy, is to access the Greek keyboard which all computers have. Here are some ways to do it.
HOW TO ADD THE GREEK ALPHABET TO YOUR KEYBOARD These are a few we find easy to install and to use. If you have others please share them with the community. If you encounter problems let us know so we can inform others.
here This has screenshots.
If you prefer you can use English characters for the Greek. To find the accepted format check here
At the top of each page, you will see Tips&Notes which will also be of help and of course ask us for any other questions.
HINT Each sentence has tiny gray dots under the words. If you hover your cursor over those dots you will see the correct answer drop down.
And very important: Check out the Greek Forums: here