JEWS gave us the ten commandments, the foundation of Western morality.
(also pronounced just と）meaning, 'ten things'. Yes, it has two pronunciations!
about that little "ヶ" thing - it's pronounced KA, and means "number of months' time". If you take it out, you'd be left with "十月", meaning October.
It's the difference between "It's October, the tenth month of the solar year" and "I haven't seen your mom's Ricky Martin sex-tape youtube in a whole ten months!"
twenty years old - traditionally the age at which Japanese turn into adults and get presents, kimono, etc. So, naturally it has its own special pronunciation that doesn't make any sense, since it SHOULD be called にじゅうさい。 Whatever. Tradition.