1) The child drew a square in the sand, but the ocean erased it.
2) The child drew a square in the sand, but the ocean has erased it.
Simple past : The past simple tense is an action that began and ended at one particular time in the past.
Present Perfect: It indicates either continued action or the continued effect of the completed action.
so in the (1) sentence, the ocean erased the sand and, the sand is square free. Is that right?
I am not able to differentiate between simple past and present perfect, please explain.
The present perfect is a tense that is used for something within a given period of time but still in the past.
"Yesterday I ate lunch." Since it is no longer yesterday, you use the simple past. It is always wrong to say: "Yesterday I have eaten lunch."
Today I have eaten lunch. It is still today, so you use the past perfect. It's fine to say, "Today I ate lunch." It's about proximity in time. At 1pm, my coworker might approach me at say, "Have you eaten lunch yet?" But when I get home, my boyfriend will say, "Did you eat lunch today?"
Sentence 2 just gives the feeling that the "erasure" happened more recently.
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Yale School of Management MBA Candidate, 2015.