The (In Welsh):-
• If the word before (the), ends with a vowel, it has " 'r " added to the end of that word, regardless of what comes after it.
So, if the word before doesn't end with a vowel, then:-
• "Y" is used before a word beginning with a consonant.
• "Yr" is used before a word beginning with a "H" or a vowel. ("W" and "Y" are also vowels in Welsh.)