Learn English – A new study [shows / showing / is showing / showed] that

1: Is my understanding of this construction correct?

Yes, but it's slightly more general than that. The tense used on both sides depends on whether that event has happened. For before the until, the tense is used as normal. For after the tense is past if it has happened and present if it has not.

• Example 1: try to think of the author as writing from the point of view where the officials are waiting for a surge. They have already been waiting for one, but one has not happened yet. Hence, perfect + until + present.

• Example 2: the person has already done something they're unaware of but don't yet have the descriptions right. Hence, perfect + until + present.

• Example 3: they have shot ** but **have yet to process. Hence, perfect + until + present.

• An extra example: 'I will wait until he arrives'. The person has not started waiting and he has not arrived.

• An extra extra example: 'I'd waited until he arrived'. The person has already waited and he has already arrived.

2. Is this a common construction? Does it apply to other words besides "until"?

Until is a common word in English, and it's almost always used in this fashion. It's an action that ends upon a condition, where the action is before the 'until' and the condition is after the 'until'.

No words that act in the same way come to mind, so I can't answer that question. I can show you the word since though.

We've been gazing at the stars since he arrived: this is basically the opposite of 'until'. This defines an action that has started after a condition is met, instead of stopping. The tenses work almost exactly the same way.

To avoid confusion:

• Action has started and continues UNTIL a condition is met, then action stops.
• Action has started SINCE a condition is met, and action continues.

The simple present is generally for actions or events that you do, or often do, or for discrete events that you repeat over time. The present progressive is generally for actions or events that are going on right now.

For clarity, let's replace "to see" with "to visit" because that's the implied meaning in this context. Right now, as you write, you aren't in the process of visiting your brother. You simply visit him from time to time. So the simple present tense is sufficient. However, your brother is in the process of doing his degree. When talking about your brother's situation, the present progressive is appropriate.

This is why the correct answer has "mixed" tenses, because you essentially have two different but connected sentences, each with its own subject and verb.

Other examples of mixed tenses:

My mother is cleaning the house, so I can't play video games.

His roommate is studying for exams, but Bill still has to practice his drumming.

The opposition is saying terrible things, but the candidate still fights on until election day.