Also, I think, the gerund "walking" (the form that acts as a noun, as in "walking is good for you").
The form is often called a "verbal noun" or "verb noun" in Celtic linguistics as it has several noun-y properties. (For example, "my walking" in the sense "(the fact) that I walk" is formed with the same possessive construction as "my shoe".)
This is a good explanation. It's a verbnoun because sometimes it has verbal properties and sometimes more nominal.
Interestingly, with phrases formed with possessive pronouns in English, the pronoun refers to the subject: "my eating, my seeing, my hearing". In Celtic languages the pronoun usually refers to the object: fy mwyta i, fy ngweld i, fy nghlywed i (eating me, seeing me, hearing me).
Yep assuming you are using the "Dwi'n (past participle form). There are also ways of changing the ending of verbs to the present tense e.g Rhedeg=run(Infinitive) becomes "Rhedaf i" for "I am running". These are rarely used though unless they are in phrases that are used often e.g the "wch" ending in "Os gwelwch chi'n dda" is the present conjugation for the formal you. Don't worry about these though I'm half way through my first language Welsh GCSE in school and the only reason I know them is because I asked my teacher. (EDIT: The forms I said weren't used much are actually still used in the south