My book says that at atmospheric pressure ice crystallises in the hexagonal form but at very low temperatures it condenses to cubic form.
Why and how does this happen?
First, a little clarification. The kind of ice you see at ambient pressure, ice I, exists as stacks of bilayers of water molecules. If they are stacked in an ABAB sequence, they form hexagonal ice, and cubic ice in an ABCABC sequence. The difference between the two allotropes is just the stacking of the molecules, so in fact they are structurally quite similar. The structure of cubic ice is analogous to the diamond structure of carbon, with hexagonal ice begin analogous to lonsdalite. This picture is complicated by the fact that ice is generally proton-disordered, i.e. the molecules can have different orientations that break the crystal symmetry, subject to certain rules (the "ice rules").
There is evidence that ice crystallises as cubic ice in the upper atmosphere, namely the angle subtended by solar haloes and the angles formed in nascent snowflakes, for example. However, more recent evidence suggests that cubic ice doesn't not exist as a pure phase, but only in stacking-disordered hexagonal ice (http://pubs.rsc.org/en/content/articlelanding/2015/cp/c4cp02893g#!divAbstract). Curiously, there is also evidence that the lonsdalite form of carbon doesn't exist in reality, and is just stacking-faulted diamond (http://www.nature.com/articles/ncomms6447).
This is all very well, but does not tell us why hexagonal ice forms in preference under certain conditions, or cubic ice (?) under others. If you were to perform density functional theory calculation (essentially approximately solve the Schrodinger equation for a many atom system), you would find that hexagonal and cubic ices have almost identical energies, and are essentially degenerate at zero temperature, which makes it even more confusing why hexagonal ice is almost exclusively seen on Earth. I don't think there is a convincing answer to your question, since the best calculations do not show why the hexagonal structure of ice is lower in energy than cubic, but it could be due to a number of reasons: orientational disordering, which complicates modelling ices, zero point energy or free energy considerations, or a breakdown in the approximations used to perform such calculations. In any case, it looks like cubic ice doesn't exist under terrestrial conditions.
This is a difficult question. Apparently, researchers don't know yet (or I didn't manage to find an article which completely solves the problem).
The preference for cubic structure at low temperatures was long thought to be caused by a lower liquid-ice surface tension in cubic ice nucleus (see G. P. Johari, J. Chem. Phys., 2005, 122, 194504). However, some authors point out that this assertion is not supported by strong evidence, and note that thermodynamically speaking, there is no reason why cubic should be preferred over hexagonal (E. B. Moore and V. Molinero, Phys. Chem. Chem. Phys., 2011, 13, 20008), that is to say that cubic ice is not more stable. These authors performed molecular dynamics simulations, and concluded that a the cubic ordering is kinetically controlled, though they left the elucidation of the kinetic factors an open question.
Of course, this dates back to 2011, any update would be welcome (it is not exactly my field of expertise...).
[PS. I do not know much but I'll still try to answer based on my logic and my prior knowledge]
Temperature is the average kinetic energy of the molecules/particles in a compound/substance. At very low temperatures, the kinetic energy of the molecules tend towards 0. Also when a material freezes, the intermolecular bond length between 2 water molecules would decrease hence condensing the entire molecule.
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If your question was about the shape of a bigger ice, I think that the shape of the ice also depends upon the container than you put the water in to freeze.