Divide a number by $0$ I don't know much about maths, I just started thinking about this the other day and would like to ask the experts.

I believe in maths it is said that $\frac 10$ is 'undefined'.

This may be because mathematicians look at it like this:

$$y = \lim_{x\to0}\frac 1x $$ is a very big number, because you can subtract a very small fraction from $1$ many many times before you get to zero.

I think division is actually subtraction. 

$$1 - 0 = 1$$

Now, maybe mathematicians say, "How many times can you subtract $0$ from $1$? We can't say."

Teacher: Johnny, if you have $1$ apple and I take $1$ apple from you, how many apples do you have?
Johnny: None.
Teacher: Correct Johnny. Now, if you have $1$ apple and I take $0$ apples from you, how many apples do you have?
Johnny: Then you didn't do anything and I still have $1$ apple. Stupid question if you ask me.

If you 'subtract $0$' you have in fact done nothing. If you do nothing to $1$, $1$ does not change. 

$$\frac 10 = 1$$
You seem to think of division as repeated subtraction.  That's fine, it turns out there's a way to think of it like that.

When you divide $a$ by $b$ you're essentially asking "how many times can I subtract $b$ from $a$ until I get $0$?"  For instance $10/5 = 2$ tells us that we can subtract $5$ twice from $10$ to get $0$.  $-4/2 = -2$ tells us that we can "subtract" $2$ negative two times to get $0$: but it's easy to see that that just means add $2$ twice.  It's a little harder to see what something like $4/5$ would mean: you'd have to subtract $5$ from $4$ a total of $0.8$ times to get $0$.  But somehow that sort of makes sense because you couldn't possibly subtract all of one $5$ from $4$ or you'd overshoot.

Now we turn our attention to $1/0$.  How many times do we have to subtract $0$ from $1$ until we get $0$?
Only 0/0 is 'undefined', because any number multiplied by 0 leads to 0. Any number divided by 0 tends to infinity, because the lower the divisor, the higher the result (if the dividend is different form 0, of course).