Huckel's rule and aromaticity I am learning to determine whether a monocyclic conjugated alkene is aromatic or anti-aromatic or not.
question 1:
cyclobutadiene: according to Huckel's rule, 4 * 1 = 4, it should be antiaromatic, like what is said here. However, Wikipedia says the 2 pi bonds have a 'more double bond' character, making it a rectangular shape (electrons not delocalized). Therefore, it is more like a conjugated diene, hence non-aromatic. What is the truth?
question 2:
planar / non-planar: for [18]annulene, it is aromatic (4 * 4 + 2 = 18). From Wiki,  ''it is large enough to comfortably accommodate six hydrogen atoms in its interior, allowing it to adopt a planar shape, thus satisfying Hückel's rule.'' So in this molecule, there is an extremely small steric effect. So I am wondering, is size of the inner space also a factor in determining whether a cyclic molecule is aromatic or not? If the inner lumen of the molecule is larger, it can follow Huckel's rule? Like Cyclodecapentaene, as the 2 hydrogens cannot be in planar (not comfortably accommodate) (figure 3 in Wikipedia). It is non-aromatic rather than aromatic (do not follow Huckel's rule).
question 3:
cyclooctatetraene is non-aromatic rather than antiaromatic (4 * 2 = 8) since it avoids by changing into a 'tub' shape (avoiding being the unstable planar conformation). I am wondering if other supposedly antiaromatic molecules can change their conformation to be non-aromatic (become more stable) as cyclooctatetraene does? Are there any criteria for changing the shape, or cyclooctatetraene is the only example? If yes, why? If no, what monocyclic conjugated alkene molecule is actually antiaromatic then?
Thank you very much.
Steric effects play an important role in determining the planarity, aromaticity of a molecule. Cyclobutadiene is sterically unstable(90 degrees angle between bonds), hence having high energy, and thus  anti-aromatic.
A lot of compounds change their conformations for reducing their overall energy. Check for the conformations of glucose, for instance. A compound will always prefer its low energy conformation, which occurs when Cyclooctatetraene is non-planar depicted in the right hand perspective drawing

(reference)
This compound looks like the big brother of benzene, which may incorrectly induce the notion of it being aromatic, or, by Huckel rule, anti-aromatic. But the inner angle of a regular octane shape is 135 degress, while a much stable angle nearby it is 120 degrees(an sp2 config.), which only occurs in the tub config. of Cyclooctatetraen. 120 degrees is not achieved, but ~126 degrees is achieved.
Frozen (at 129 K) cyclooctatetraene's structure was characterized by single crystal diffraction analysis by Claus and Rüger. You find the corresponding interactive model here.