Search, then stop

You look through your bag for your keys. The moment you find them, you stop looking. Searching a list works the same way.

This time you want the position, not the value, so loop over the positions. Look at the item in each box. As soon as one passes your test, return its position straight away. You already know the answer, so there's no need to look at the rest.

Found it? Return its position at once.

In code

for i in range(len(nums)):
    if nums[i] < 0:
        return i

The first negative number

-2

i: 0
x: 5

Position 0 holds 5. It's not negative, so keep looking.

Move 1 of 4

When it isn't there

What should the function return if nothing in the list passes the test? You can't return a position, because there isn't one. Programmers have a habit for this: return -1.

That works because no real position is ever negative, so -1 can't be confused with an answer. Put the return -1 after the loop. If the loop finishes without returning, nothing was found. Whoever calls your function knows what -1 means.

No match? Return -1, a position that can never be real.

In code

for i in range(len(nums)):
    if nums[i] < 0:
        return i
return -1