。
When I looked outside, I saw it had started raining.
Literal
Outside [object-を] see [conditional-と] rain [subject-が] fall-start [progressive-ている].
~と as a temporal-automatic conditional ('when X, then Y inevitably') often signals a discovery: 'when I did X, I found Y.' This is one of the most useful and subtle functions of the と conditional. ~始める attached to a verb stem forms a compound meaning 'start to X.' The ~ている form marks the ongoing state of having started.