Can a number have infinitely many digits before the decimal point? I asked my teacher if a number can have infinitely many digits before the decimal point. He said that this isn't possible, even though there are numbers with infinitely many digits after the decimal point. I asked why and he said that if you keep adding digits to the left of a number it will eventually approach infinity which is not a number and you could no longer distinguish two numbers from one another.
Now this is the part of his reasoning I don't understand: why can we distinguish two numbers with infinitely many digits after the point but not before it? Or is there a simpler explanation for this?

It's a matter of semantics and definition.  Technically, since integers are a subset of real numbers and there are an infinite number of them, you can have an infinite number of digits; however, for the sake of defining properties, and defining numeric fields, you limit the number of digits to n... which can be as large as you like.
So, yes, you CAN have an infinite number of digits - because you can always make your number larger by multiplying it by 10, say - but, for the sake of definition and math theory, it is NOT possible, by nature of its definition.
