By then can be taken to mean “when the time under discussion has arrived” so we expect that what will follow is something that will be true at that point.

You will show is indicating that you are going to show something in the future.

Both things don’t mesh: you can’t talk about unspecified future event happening in relation to a specific point in time that is under discussion. Why also makes this clear.

When you will have shown what you meant to do since the beginning, then you will show how you can have achieved your goal in life.

The future perfect you would have had

is saying we are looking back from some point in the future where you would have already shown what is being talked about. By then tells the future what is going to be. So the sentence make perfect sense.

By then can be taken to mean “when the time under discussion has arrived” so we expect that what will follow is something that will be true at that point.

You will show is indicating that you are going to show something in the future.

Both things don’t mesh: you can’t talk about unspecified future event happening in relation to a specific point in time that is under discussion. Why also makes this clear.

When you will have shown what you meant to do since the beginning, then you will show how you can have achieved your goal in life.

The future perfect you would have had

is saying we are looking back from some point in the future where you would have already shown what is being talked about. By then tells the future what is going to be. So the sentence make perfect sense.

By then can be taken to mean “when the time under discussion has arrived” so we expect that what will follow is something that will be true at that point.

You will show is indicating that you are going to show something in the future.

Both things don’t mesh: you can’t talk about unspecified future event happening in relation to a specific point in time that is under discussion. Why also makes this clear.

When you will have shown what you meant to do since the beginning, then you will show how you can have achieved your goal in life.

The future perfect you would have had

is saying we are looking back from some point in the future where you would have already shown what is being talked about. By then tells the future what is going to be. So the sentence make perfect sense.

By then can be taken to mean “when the time under discussion has arrived” so we expect that what will follow is something that will be true at that point.

You will show is indicating that you are going to show something in the future.

Both things don’t mesh: you can’t talk about unspecified future event happening in relation to a specific point in time that is under discussion. Why also makes this clear.

When you will have shown what you meant to do since the beginning, then you will show how you can have achieved your goal in life.

The future perfect you would have had

is saying we are looking back from some point in the future where you would have already shown what is being talked about. By then tells the future what is going to be. So the sentence make perfect sense.

By then can be taken to mean “when the time under discussion has arrived” so we expect that what will follow is something that will be true at that point.

You will show is indicating that you are going to show something in the future.

Both things don’t mesh: you can’t talk about unspecified future event happening in relation to a specific point in time that is under discussion. Why also makes this clear.

When you will have shown what you meant to do since the beginning, then you will show how you can have achieved your goal in life.

The future perfect you would have had

is saying we are looking back from some point in the future where you would have already shown what is being talked about. By then tells the future what is going to be. So the sentence make perfect sense.