I have been thinking about this and trying to come up with some way to rearrange these tags and honestly I don't think this is a good place to start with the clean-up. Each of these tags when applied to a question is meaningful, even though there is some redundancy. I think it would be better to find tags that are applied to questions that are so different they shouldn't be grouped together (like letters questions before correspondence and letters-of-the-alphabet) or to find questions for/remove tags that are only applied to a few questions or to add more specific tags to questions that just aren't grouped at all (like the ones that have just the grammar tag).
I think if we look at the tags without looking at the questions they're applied to, we would find lots of cases like the aspect/tense tags where it seems really messy and in need of fixing, but I don't think that sort of clean-up would get us a lot of bang for our buck. I would like to take a different approach that comes from the perspective of finding questions better tags.
For example, we could look at the questions tagged with continuous and see if we can't get them folded into a more specific tag that has some guidance on what sorts of questions it should be applied to. If we find that it's difficult to find proper tags for these questions, then we should figure out if we need to make new tags, or make some synonyms.
I think by focusing on a set of questions instead of a set of tags we can simultaneously improve our question categorization, learn by doing which types of questions should be tagged with certain tags, and can work on continuously improving the site instead of a few folks burning themselves out with massive retagging efforts that don't really "stick".