This is the first I'm seeing tea spelled "the" instead of "te". Why is it changed?
This is because of the word 'na' introducing one of the mutations into 'te'
It's called an aspirate mutation and it's the least common of the three, it only affects the letters 'C', 'P', 'T'
There are loads of notes online about Welsh mutations and also in the unit 'Ga i' here.
But, don't worry about leaving them out when speaking.