The value of √-16 is not - 4 because

Square - 4 by multiplying it with itself

(-4) ^2 = - 4 times - 4 = 16

The result is positive. So its not possible to square a negative number to get a negative result. This is why taking the square root of a negative number isn't possible.

Note: the notation i = sqrt (-1) is used to circumvent this, but you'll probably learn about that later