had a toothache

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(a) He has a toothache so he went to see a dentist.-- Possible if he is now en route to the dentist

(b) When he has a toothache he will see the dentist.-- OK

(c) He had a toothache so he went to see / visit the dentist yesterday.

(d) He had a toothache, so he visited the dentist. -- OK. ' Visit' is marginally appropriate

Thanks. But.

a) He has a toothache so he went to see a dentist.
("has" is present, "went" is past tense, it can exist in a sentence?)

(c) He had a toothache so he went to see / visit the dentist yesterday.
(If got "had", can we use time frame, "yesterday"?)

a) As Mr. M said, if he is on his way to the dentist, he still has the toothache (has) but he already left to go to the dentist (went).

c) both "had" and "went" are in the past. They both work with "yesterday."

If you said the sentence is correct, like:

c) both "had" and "went" are in the past. They both work with "yesterday."

Can I say,

She had read the books for two hours yesterday. (isn't correct? )

.
She had read the books for two hours yesterday. (isn't correct? )-- This is not correct without additional context including at least another past event which the book reading precedes.
.

Don't confuse "had" as the simple past of the verb "to have" with "had" used to form the past perfect.

I have a toothache right now.

I had a toothache yesterday.

I have had a toothache for two days.

I had had a toothache for two day before I finally went to the dentist.