Also, how can you have a negative pH value??

Brain is exploding, is this possible for a pH value over 14 as well?

Lets think about it mathematically. pH=-log(H+) and subsequently 10^-pH =H+ concentration. If the H+ concentration is 1.5M, -log(1.5) will be equal to -0.176. The negative sign in front of the log makes it confusing, but if you were to divide both sides by -1, the equation would become 0.176=log(1.5), it can now be seen easier that 10 to the power of what is equal to 1.5. Another way we can look at it is 10^-pH = H+, subbing in the pH as -0.176 will cancel the negative and the equation will become 10^0.176 = H+. It may be beneficial to ask your maths teacher when you get back, as logarithms and exponentials play a big role in methods and they may be able to explain it a lot better.

The same does apply in the higher end, you can have a basic solution that has a pH above 14.