Learn Dutch: Als ik de loterij won, zou ik heel rijk zijn. - If I won the lottery, I would be very rich.

Just like in English (If I won), Dutch uses the simple past tense here to show that the situation is hypothetical or unlikely. Even though you are imagining a situation right now or in the future, you use the past tense won after als to express this fantasy.

Why is the word order zou ik and not ik zou in the second half?

This is the V2 (verb-second) rule in action. The entire if-clause (Als ik de loterij won) counts as the first block of information. To keep the main verb in the second position, zou must come right after the comma, followed immediately by the subject ik.

What exactly is zou and where does it come from?

Zou is the singular past tense of the verb zullen (will/shall). In Dutch, combining zou (or the plural zouden) with an infinitive at the end of the sentence (here, zijn) is how you create the conditional 'would' form.

Can I flip the sentence around and start with the main clause?

Yes! If you put the main clause first, it uses standard word order: Ik zou heel rijk zijn als ik de loterij won. Notice that in the subordinate clause (als ik de loterij won), the verb won still gets pushed to the very end.