The obvious difference in your examples is that "which" is a relative pronoun, whereas "what" is a determinative. [1] I saw a strange craft i [ which i appeared to be a UFO ]. [2] I saw [ what appeared to be a UFO ].
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The obvious difference in your examples is that "which" is a relative pronoun, whereas "what" is a determinative.
[1] I saw a strange crafti [whichi appeared to be a UFO].
[2] I saw [what appeared to be a UFO].
[1] is a straightforward relative construction where the bracketed element is a relative clause in which the pro