[Vocabulary] "he understood" or "did he understand"?

[marin123]

Answer, please, which variant is correct:

Only after I explained it to him he understood the problem.

or

Only after I explained it to him did he understand the problem.

I believe the second varient to be correct as an emphatic one, but I'm not sure. Please, correct me if I'm wrong.

 

[bhaisahab]

Answer, please, which variant is correct:

Only after I explained it to him he understood the problem.

or

Only after I explained it to him did he understand the problem.

I believe the second variant to be correct as an emphatic one, but I'm not sure. Please, correct me if I'm wrong.

After I explained it to him he understood the problem.
Only after I explained it to him did he understand the problem.
These two are correct.

 

[~Mav~]

It has just called to my mind a line delivered by Emperor Palpatine from "Star Wars: Return of the Jedi" when Palpatine wanted to "electrocute" Luke Skywalker, and after the first jolts of lightning, he told him:

"Young fool... Only now, at the end, do you understand..." (It's right after the beginning of the video, at 0:15.)

Classic!