Positive statements are awkward. We always mutate negative statements and questions, but for positives you have a couple of choices. Firstly, I'll explain why. Traditionally Welsh used a lot of different verbal particles. These pretty much have no meaning other than saying that somebody is doing something and if it is positive or negative. The three main ones were "Mi/Fe" (positive statements "Mi" being northern and still widely used), "Ni" (for negative statements) and "A" (for questions). I'll just carry on with "mi" since that's the one at work here. So, in modern Welsh you can either say this sentence using "Mi fasai'n", "Fasai'n" or "Basai'n". This is because if you include the "mi" it causes a soft mutation, if you leave it out you can mutate it since traditionally there was a reason to mutate or you can not mutate since there is nothing there to cause a mutation.
Also one last note, you mention that in negative statements you have a soft mutation. This is true to some degree as the negative particle caused what's called a mixed mutation. This sounds worse than it is, but basically T, C and P undergo aspirate mutation in negative statements but all the other letters that can mutate mutate softly in negative statements. e.g "Chlywais i ddim" (I didn't here, with an aspirate mutation) and "Faglais i ddim" (I didn't trip with a soft mutation).