Khullak: There's nothing wrong with starting a sentence with "there is" or "there are." This is called the "existential there," in which "there" is sort of standing in for another noun. "Cars are in the street" vs. "There are cars in the street." Sometimes this is an inefficient way to make a point, which is how the rumor got started that this form is wrong. But in fact, this form is perfectly grammatical, if not always ideal.
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]]>Tom: Good question. Any of those options is fine. There's nothing wrong with having more than one tense in a sentence if that best conveys the meaning. "She is the one I loved." This might be best if you're pointing across the room at a woman whom you no longer love but who's standing there right now." In our sentence, both "It is John" and "It was John" are fine, though the difference is much smaller than in the "love" example. The choice depends only on whether you want the emphasis to be in the past or in the present.
]]>Is it? Aren't the tenses mixed? Shouldn't it be, "It is John who is eating the last piece of cake"? Or "It was John who ate the last piece of cake"?
]]>Gab: "It" is special. It doesn't always get an antecedent. Sometimes, according to Merriam-Webster's, "it" is "used as subject of an impersonal verb that expresses a condition or action without reference to an agent: 'it is raining.'"
Other times, "it" can have antecedents that are a little murky. Like in "I'm looking at plane fares for Josie's wedding but I'm not sure I'll go. It depends on whether I get the job." What depends? Well, something that wasn't stated outright but is still clear to the reader: "the question of whether I'll go to the wedding."
So "it" can have an express or not-so-express antecedent, or it can function in its unique job in which it requires no antecedent "It is raining."
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