O que significa I had opened a restraunt. ?

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This is called the Present Perfect tense.

"I had opened a restaurant" means it happened sometime in the past. You can't say "I had opened a restaurant last year." If you say when it happened, you can't say it "had" happened.

It doesn't always mean that the restaurant closed, or the action was finished. In fact, it is often used in sentences in which something else happened after you opened it, or something kept happening after you opened it.

"I had opened a restaurant when I realized I wanted to be a lawyer."
"I had opened a restaurant and I was working 12 hours a day."
"I had opened a restaurant, but I was forced to close it because of personal issues."

You might see something like this sentence: "It was 1990, and I had opened a restaurant."

But you aren't supposed to say a specific time, right? The reason a sentence like this is okay is that you didn't open the restaurant in 1990. You had already opened the restaurant sometime before 1990.

You can also use the Past Perfect tense to talk about something that is still happening. "She has had the flu since Tuesday."

Note: Even though you can't use the Present Perfect if you specify a time, sometimes you can use the Past tense without specifying a time. Example, "I opened the door." The Past tense does mean the action was finished. But in your example, it doesn't necessarily mean that the restaurant was closed. You can say, "I opened a restaurant," meaning you finished opening the restaurant. Sometimes the context of the conversation tells you when it happened.

"What did you do last year?"
"I opened a restaurant."

"What did you do last year?"
"I got sick and went to the hospital. I had opened a restaurant and I was working too hard, so I was forced to hire a manager."