Learn German: Die Bar öffnet erst um zehn. - The bar does not open until ten.

In German, you cannot use a negative phrase like nicht bis for points in time. When an event happens later than expected, you use the positive adverb erst. So, erst um zehn is the standard way to express 'not until ten'.

Could I use nur instead of erst if I wanted to say the bar opens 'only' at ten?

No, because nur refers to a quantity or limitation (like having 'only one beer'). When 'only' refers to a point in time or a delayed start, you must use erst. Using nur here would sound confusing to a native speaker.

Is there a difference between using öffnet and aufmacht in this context?

Both mean 'opens', but öffnen is slightly more standard and formal. In casual spoken German, you will frequently hear the separable verb aufmachen, as in Die Bar macht erst um zehn auf. However, öffnet is still completely natural for stating official business hours.

Is it always okay to drop the word Uhr after zehn?

Yes, in everyday communication! Just like in English where you can say 'at ten' instead of 'at ten o'clock', Germans often drop Uhr when the context makes it perfectly clear that you are talking about time.