Learn Swedish: Föreställ dig att du vinner en miljon.

The verb föreställa sig (to imagine) is reflexive in Swedish, meaning it always needs a reflexive pronoun. Even though English simply uses "imagine", in Swedish you must include it. Because this is an imperative command directed at a singular "you", the correct pronoun is dig.

Could I use tänk dig instead of föreställ dig in this sentence?

Yes, tänk dig att du vinner... is another very natural way to express this. Both föreställa sig and tänka sig mean to picture something in your mind or to imagine a scenario.

Is it possible to use the conditional skulle vinna instead of the present tense vinner here?

Yes, you can say att du skulle vinna (that you would win) to highlight that it is a purely hypothetical situation. However, using the present tense vinner is perfectly correct and very common in Swedish, as it makes the imaginary scenario feel more vivid and immediate.

Does en miljon follow the same number rules as hundreds and thousands in Swedish?

Unlike hundra (a hundred) and tusen (a thousand), which act as numbers and drop the article, miljon is a standard noun. This means it requires an article, so you must say en miljon just like you say "a million" in English.