Inversion

[Ivonne28]

Hello,
I did the testing on the inversion and was shocked, because I made a lot of mistakes.
Could anyone please explain to me the rule, while it is "Seldom have I seen such a mess" but on the other hand it is "Sometimes I have seen such a mess"
Thank you in advance!!!

[Tdol]

Not all adverbs invert- they tend to be negative or nearly negative ones, where there can be an emphasis- it's hard to emphasise 'sometimes', which is a pretty meaningless adverb at the best of times.