If I enchanted a sword with a level thirty enchantment what chance would i have to get fire aspect I or II or sharpness V, is it a definite or is it unpredictable?
It is not predictable, what enchantments you will get but at certain enchantment levels, you will get some enchantments with a higher probability. This chart shows the probability for diamond tools:
Also, the Minecraft Wiki can give you the algorithm Minecraft uses for calculating enchantments. It is pretty complex and you really don't need to get into it, but it is definitely interesting: How Enchantments Are Chosen
The minecraft wiki offers a very detailed description of what enchantments you'll be able to get but the short version is:
With regards to Sharpness and Fire Aspect, Sharpness is pretty likely (about 1 in 3 chance of it being applied the first time), however it conflicts with the other damage enchantments (Smite and Bane of Anthropods) for which you also have about a 1 in 3 change of them getting picked (either one being picked is 1 in 3).
Fire aspect is less likely, it only has a 2 in 29 chance of being applied as the first enchantment.