1. Only then did I understand the problem.

Question

2. Only then I understood the problem.
(Which one is grammatical?)

3. Only after understanding the situation did he make a comment.

4. Only after understanding the situation he made a comment.

5. Only after understanding the situation, did he make a comment.

6. Only after understanding the situation, he made a comment.

(Which ones are grammatical?)

Answer 1

#s 1, 3, and 5 are correct if you remove the comma in 5!