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Consider the following statement:
If I were you, I would go to the doctor.
As far as I know, a modal verb or a past tense can be interchangeably adopted to express the subjunctive mood. It seems, however, that most subjunctive sentences commonly prefer to use a modal verb, rather than a past tense. In conclusion, my questions are
Is the following sentence grammatically correct?
If I were you, I went to the doctor.
If so, what is the reason most people prefer to use a modal verb?
It's important here to distinguish between the preterite form of a verb, and the past tense meaning that is normally associated with the preterite form. The modal preterite use of some English verbs indicates modality, as the OP points out (not past time). For example, in
I wish you went to the doctor more often than you do.
...or..
If you went to the doctor tomorrow, I'd drive you.
...use of the preterite form of the verb (went) does not indicate past time.
To answer your question (why you can't say "If I were you, I went to the doctor"), English does not grammaticalize a generic concept of modality (nor does it have a robust grammatical category "subjunctive" as French does), but instead has several common constructions that can be used to convey different types of modality. The modal preterite construction cannot be used in the same situations as a modal auxiliary can. The modal preterite expresses modal remoteness, generally in one of the two types of constructions:
Auxiliary verbs can be used to convey a larger range of modal meanings (with different auxiliaries having different modal meanings associated), some of which include remoteness. In a sentence like "I would go to the doctor (If I were you)," would expresses two types of modality: propensity (lexically) and remoteness (preterite form).
“If I were” is not past tense, it is subjunctive and refers to present or future time and is about a contrary to fact or hypothetical idea. The way to express the result clause is with “would” instead of “will.”
For example, real condition: If I am there, I will see you. Unreal/hypothetical/contrary to fact: If I were there (=I am not), I would see you (=I don’t see you or won’t see you because I’m not there.)
Sadly the present tense subjunctive is technically called “past subjunctive” in many authoritative sources so this causes some confusion. It is about present time. Past (time) subjunctive would be: If I had been there (=I wasn’t), I would have seen you. (=I didn’t see you.)
