1) Statement: We came here.Originally Posted by Anonymous
2) Question: Why did we come here?
In 1), the verb "came" is in the past tense, and in 2), the verb "come" is in the present tense because the auxiliary "did" carries the past tense:
Present tense: come, do
Past tense: came, did
Step 1 Input: We came here.
Step 2 WH-insertion: Why we came here?
Step 3 DO-insertion: Why do we came here?
Step 4 Tense transfer: Why did we come here?
Step 5 Output: Why did we come here?
The first verb always carries the tense: "did" carries the past tense and "came" becomes "come". :D
Hope that helped. :D