At least in Hiberno English, there is very little semantic difference between "I used to see that man every day" and "I would have seen that man every day" - unless there was an "if" or something else in the statement to force the conditional, it often wouldn't matter whether you wrote it as D'fheicinn or D'fheicfinn.
I think that there is a slightly subtle difference between the two, at least in American English (could be other English's as well :)), as exemplified by the following:
1 - Have you seen the bears at the zoo? 2 - Yes. I used to see the bears at the zoo when I was little. 1 - What about the last time you went? 2 - Yes. I would have seen them the last time I was there.
"Used to see" seems to have a more continuous feel while "Would have seen" feels more specific.
I can see how many people would interchange the two, but there does seem to be a distinction, at least in esoteric hypotheticals. :D
…it often wouldn't matter whether you wrote it as D'fheicinn or D'fheicfinn.
Perhaps I’m not fully understanding your point, but I don’t think I agree. It’s true that “She would see the man everyday” and “She used to see the man everyday” can be closely aligned in English, but this is because “would” can carry either past-habitual or conditional meaning. The sentence is ambiguous. Irish however keeps the meanings distinct. Consider:
-D’fheiceadh sí an fear sin gach lá
-D’fheicfeadh sí an fear sin gach lá
“She would see that man everyday” is definitely a suitable translation for both of these sentences, however the sense of the two are quite different from each other. The first sentence, using the past habitual tense, is retrospective and more-or-less identical in meaning with “She used to see the man everyday.” It’s also clearly factual, a description of events that definitely happened.
The second sentence, by contrast, is prospective. It references something that might happen in the future. Another possible interpretation, since the Irish conditional is tenseless, is “She would have seen that man everyday.” Under either of these possible readings there’s an element of uncertainty or doubt that isn’t present in the first sentence.
So the meaning of D’fheiceadh and D’fheicfeadh are definitely distinct, and it definitely matters which you write provided you have a clear idea of the meaning you’re trying to express. The only time it doesn’t matter is when the sense of the sentence is unclear. There may be instances where this is the case, especially on a platform like Duolingo, but in actual settings this would be pretty rare.
(This probably doesn’t apply to Ulster Irish, where the past habitual has fallen out of regular speech. As far as I understand it’s been replaced by a combination of a periphrastic construction and the conditional mood)