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A. What he did is correct.
B. What he did was correct.

1. Does sentence A mean that what the person did still holds true up to the present?
2. Whereas sentence B may either be interpreted as true until today or no longer true?

C. What he did is he watched the movie.
D. What he did was he watched the movie.

3. Does sentence C with the linking verb 'is' emphasize that what the person did is a statement of fact that he did watch the movie?
4. Is sentence C gramatically acceptable or possible in the first place with the reason given in #3? If no, therefore sentence D is the correct one.
5. In sentence D, is it 'What he did' that determines the correct tense of the linking verb or is it 'he watched the movie'?
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1. Does sentence A mean that what the person did still holds true up to the present?-- Yes

2. Whereas sentence B may either be interpreted as true until today or no longer true?-- Yes

3. Does sentence C with the linking verb 'is' emphasize that what the person did is a statement of fact that he did watch the movie?-- Not for me.

4. Is sentence C grammatically acceptable or possible in the first place with the reason given in #3?-- It's grammatically correct but does not do what you want it to do, in my estimation.

5. In sentence D, is it 'What he did' that determines the correct tense of the linking verb or is it 'he watched the movie'?-- The subject ('What he did')
Comments  
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Mister Micawber,

Thank you for your helpful response.
Mister Micawber2. Whereas sentence B may either be interpreted as true until today or no longer true?-- Yes
Just to clarify with question #2 because I just read in a forum that 'until today' could either mean until just today or so far. Sorry for the confusion I made, but what I really meant was whether sentence B may either be interpreted as true and continuing up to the present or no longer true. Is the answer still yes?

Another similar example:

E. My previous supervisor is understanding and helpful.
F. My previous supervisor was understanding and helpful.

6. I believe sentence E means my previous supervisor is still understanding and helpful up to this moment. However, I'm not sure if the tense of the linking verb is correct because as I understand what you've explained, the subject determines the correct linking verb. The subject 'My previous supervisor' indicates past so I'm not sure if 'is' is correct here in the first place. Is my understanding correct?

7. Would sentence F mean that my supervisor in my last position is still understanding and helpful with her staff members at present?

8. Or would sentence F necessarily mean that she is no longer understanding and helpful with her staff members at present? Although I know for a fact that she still is understanding and helpful like before when I was still her staff.
9. Just an additional question, does the use of 'was' only limits the time, but not necessarily that the adjective describing the person is no longer true? For example:

When I was still with A Company, my supervisor was understanding and helpful.
(But it doesn't necessarily mean he is no longer understanding and helpful at present?)