Sentence |
Correctness |
"What subject did you taught as a teacher?" |
Bad |
"What subject did you teach as a teacher?" |
Good |
"What subjects have you previously taught? |
good |
"Which subjects have you taught in the past?" |
good |
"In the past, which subjects have you taught?" |
good |
The phrase "teach as a teacher" is a little redundant, but it is not too bad.
The word "did" is like a machine. Words in present tense are inputs to the machine. Outputs of the machine are words in past-tense.
Likewise, the word "did" is like a function in math class.
did("teach") = "taught".
Many English words (such as "did" modify modify how words appearing further to the right should be interpreted.
For example, people write "we will probably swim at the old pool tommarow."
The word "swim" is written in present tense. However the word "will" modifies the time at which things happen.
As you go from left to right through an English sentence, if you encounter the word "did", then you are supposed to change your view-point in terms of time.
Inside of a "did" block, eveything written in present tense should interpreted as happening in the past.