Tyn "tight" is an interesting word because when you add a soft mutation, you also change the spelling slightly. If you were to just use the mutation, they you'd have the word dyn "man", so you need to remember the extra n: dynn "tight".
Similar things happen with one or two other words. For example, cwyn "complaint" becomes gŵyn (soft mutation + circumflex) so as not to confuse it with gwyn "white", and also chŵyn (aspirate mutation + circumflex) so it doesn't look like chwyn "weeds".
This is a deleted sentence at the moment, if you can tell me what unit it was I'll try and find it and re-edit it.