Learn English – “My car was breakdown” / “My car had been breakdown”

This is a tricky question. The native ear will immediately recognize that "had been remained" is not correct. The had should be followed by a past participle. Remained and been are both past participles; you can use one or the other, but not both.

So, either of these could be used to start the sentence:

• He had been president for 20 years...
• He had remained president for 20 years...

This issue gets tricky, however, when you switch to the passive voice. In that case, you can use he had been followed by a past participle, as in:

• He had been elected 20 years ago...

That's a valid formation, and it's listed as the past perfect passive verb form in this table¹:

So, the question becomes, why can the verb elected be used in this way, but not the verb remained?

The key is that the sentence with elected is using the passive construction, but the sentence with remained has an active construction. As Dave Sperling says on his ESL website:

Because subjects of passive verbs receive the action, verbs that cannot have objects (intransitive verbs) do not have passive forms.

If you look up the words in a dictionary, you'll see that elect is transitive, and remain is intransitive, which is why had been remained sounds so awkward to the native ear, while had been elected sounds just fine – although many native speakers might have a hard time explaining why.

Now, you can explain it for them: "It's because remained is an intransitive verb, so it cannot be used in the passive voice."

Agree with @Teleporting Goat - "came" is correct here. "Had come" is the past perfect tense, which is used to describe something that happened before another action. It would be correct to say, "My boss came to my desk, but my coworker had come already." This means that your coworker was there before your boss. The reason why I think "had come" is incorrect is because you aren't saying that something else happened before your boss came to your desk.

According to me using Had come gives idea about fist completed action of coming to my desk and then second action of explanation.

Close, but "had come" doesn't imply this - at least, not in the original sentence. Your original sentence (with the correct verb) already makes that implication, actually; you don't need "had come" to convey that.

"My boss came to my desk and it took me 10 minutes to explain X to him."

"And" loosely implies a sequence, especially because coming to your desk seems like something that would happen before the explanation.

Some other examples:

"The player kicked the ball and scored a goal."

"I buckled my seatbelt and turned on the ignition."

Both of those sentences are technically correct regardless of the order in which the actions happened. But the implication is that the two actions happened in sequence.