I'm pretty sure whitespace in F# is one way to control blocks. So anything further indented than an if is part of the if block. I'm definitely not an F# expert though, so I'm sure there's more to it than just that.

I also think there's a way to write F# that doesn't have significant whitespace, but uses a lot more keywords. Verbose syntax, I think that's called. I almost never see examples written that way, though.