Is “had” correct English grammar?

Yes, "had" is a correct and commonly used word in English grammar. It is the past tense form of the verb "have." It's used in various contexts:

1. Past Perfect Tense: It's used to indicate an action that was completed before another past action or a specific point in the past. For example: "She had finished her work before the meeting started."

2. Past Perfect Continuous Tense: This tense indicates an action that had been ongoing for a period of time before something else happened in the past. For example: "They had been waiting for hours when the bus finally arrived."

3. As a Past Auxiliary Verb: "Had" can also be used in forming the past perfect and past perfect continuous tenses alongside the past participle or the present participle, respectively.

Yes, "had" is a correct and commonly used word in English grammar. It is the past tense form of the verb "have." It's used in various contexts:

1. Past Perfect Tense: It's used to indicate an action that was completed before another past action or a specific point in the past. For example: "She had finished her work before the meeting started."

2. Past Perfect Continuous Tense: This tense indicates an action that had been ongoing for a period of time before something else happened in the past. For example: "They had been waiting for hours when the bus finally arrived."

3. As a Past Auxiliary Verb: "Had" can also be used in forming the past perfect and past perfect continuous tenses alongside the past participle or the present participle, respectively.

Yes it is. Had is usually used in the past tense. 

The Verb form is did for had do for have and does for has. 

It can be a main Verb or an auxiliary Verb

 For example.

 I had a car. Here had is main Verb.

She had completed the work.

here had is auxiliary Verb and completed is the main Verb.

So it's correct to use Had in english grammar and they're quite important.

Hope you understood.