Die Sanduhr läuft, während wir eine Karte aufdecken.

Yes. Sanduhr is a feminine noun, so its nominative singular article is die.

• die Sanduhr = the hourglass
• plural: die Sanduhren

As in English, grammatical gender is something you usually have to learn with the noun.

In German, laufen is often used for something that is operating or in progress.

So:

• Die Uhr läuft. = The clock is running.
• Die Sanduhr läuft. = The hourglass is running.

This does not mean the hourglass is physically moving around. It means it is currently operating, with the sand flowing.

Here, während means while and introduces a subordinate clause.

So the sentence is:

• Die Sanduhr läuft = main clause
• während wir eine Karte aufdecken = subordinate clause

It shows that the two actions happen at the same time.

Because während introduces a subordinate clause, and in German subordinate clauses are normally separated from the main clause by a comma.

So:

• Die Sanduhr läuft, während wir eine Karte aufdecken.

That comma is required in standard German.

Because während is a subordinating conjunction. In German, subordinating conjunctions send the conjugated verb to the end of the clause.

Compare:

• Main clause: Wir decken eine Karte auf.
• Subordinate clause: während wir eine Karte aufdecken

In the subordinate clause, the verb moves to the end.

Because aufdecken is a separable verb.

In a normal main clause, it splits:

• Wir decken eine Karte auf.

But in an infinitive or at the end of a subordinate clause, it usually stays together:

• ... während wir eine Karte aufdecken.

So both forms belong to the same verb:

• aufdecken = infinitive
• deckt ... auf = split finite form in a main clause

Here it most likely means to turn over, to uncover, or to reveal a card.

The exact English choice depends on context:

• in a card game: turn over a card
• more generally: uncover/reveal a card

So the learner should understand that aufdecken is the action of exposing the card so it can be seen.

Eine Karte is in the accusative case because it is the direct object of aufdecken.

We ask:

• What are we uncovering?
• eine Karte

Since Karte is feminine, its nominative and accusative singular indefinite article are both eine.

So:

• nominative: eine Karte
• accusative: eine Karte

They look the same here.

It can mean either card or map, depending on context.

Common meanings:

• eine Karte = a card
• eine Landkarte / Karte = a map
• eine Speisekarte = a menu

In this sentence, because of aufdecken and the likely game context, Karte almost certainly means card, not map.

German often uses the present tense for actions happening now, just like English.

• Die Sanduhr läuft = The hourglass is running
• wir decken eine Karte auf / wir eine Karte aufdecken = we turn over / are turning over a card

German does not need a separate progressive form like English is running or are uncovering. The simple present often covers both ideas.

Yes. That is perfectly grammatical.

German allows either order:

• Die Sanduhr läuft, während wir eine Karte aufdecken.
• Während wir eine Karte aufdecken, läuft die Sanduhr.

Both mean the same thing. The difference is mostly about emphasis:

• first version starts with the hourglass
• second version starts with the while clause

It would be:

• Wir decken eine Karte auf.

That helps show the separable verb clearly:

• main clause: decken ... auf
• subordinate clause: aufdecken

So the sentence pair is:

• Wir decken eine Karte auf.
• ..., während wir eine Karte aufdecken.