The is the most important thing that will dog your every step in Sindarin. It's the reason that Sindarin is so difficult at first. But, it doesn't need to be so hard.
Mutation is where the ends and beginings of words interact. For this lesson, we'll focus on the beginnings of words. When they interact, they change into other sounds. But, the way they change is pretty predictable, and not random. Its related to Sindarin's phonetic history. So, if you're looking at a mutated word, or trying to mutate a word, you know that it could only be from or turn into a small set of other sounds. I'll list those here for you.
Vowels don't change, only consonants.
Beginning of word → can become:
The rest of the consonants don't change into anything else.
Beginning of word ← came from: