Learn Dutch: Open dat raam meteen, alstublieft. - Open that window immediately, please.

The Dutch verb for 'to open' is openen. To give a command (the imperative), you just use the 'I' form of the verb. Since 'I open' is ik open, the command here is simply open. It happens to look exactly like the English word!

Does the formal 'alstublieft' (please) change how the verb is written?

Not necessarily. In modern Dutch, the standard command form (open) is used for both informal and formal situations. Adding the formal alstublieft is enough to make it polite. You might sometimes see an older formal style that adds a T and the word u (opent u), but the basic command form used here is completely correct and very common.

Why is 'dat' used here instead of 'die' for 'that window'?

The word for 'that' depends on whether the noun is a de word or a het word. Because window is het raam, we must use dat. If it were a de word, like de deur (the door), you would use die deur.

Where does the time word 'meteen' (immediately) belong in a command?

Time words usually come right after the verb and the specific object it applies to. Here, the verb is Open and the object is dat raam, so meteen naturally goes right after them.