'Bookmarking' difficult skills
I think it would be really good if you could 'book-mark' the individual skills that you know you need to work on particularly/are struggling with. Perhaps you could programme in some way of displaying them with a symbol above them on the home page or something like that. I'm sure others would find that really useful too.
I like the idea , but as was mentioned , they can only do so much for free.
That is why I have a notepad that includes every single section of my trees. Anything I have a problem with gets put in the notepad. I create my own bookmarks with a //search term// format. Plus it is fully searchable by words or strings.
If you know which skills you need to practice but for some reason can't remember which they are, then just keep note of them yourself--say, by noting them in a computer file or in a notebook--refer to your notes when you are ready to practice, and go do the lessons. Why should Duolingo spend resources programming something you can do so easily?
When you last reviewed something and how difficult it proved to needn't have any straightforward relationship. If you merely practice a skill, it's relatively easy to get it to gold, which I assume more or less holds for its constituent words, as well. But for a skill where you're having even the slightest difficulty, this level of practicing is usually insufficient to resolve it.
Welsh, I don't believe, has the Words tab available by default, anyway. Yes, it's available to those that know the necessary trick (and have at least a few XP on a tree where it is available), but I don't think that will do much for the matter at hand.