"I want to learn Welsh."
Translation:Dw i eisiau dysgu Cymraeg.
yn/'n is used to link dw i, dych chi, etc with verb-nouns such as hoffi (liking, to like):
- Dw i'n hoffi... - I like...
- Dych chi'n hoffi... - You like...
This is explained in the notes on the next section of the course, 'Present Tense 1'. It is also used in other patterns which are explained in the notes as they come up in the course.
In the pattern that you have just learned with eisiau, eisiau is not actually a verb-noun in Welsh, so yn/'n is not used. Once you have done another section or two this summary may help.