<p>In 1972, to reduce pollution in the Great lakes" limits having been set by the United States and Canada" on the amount of phosphorus that could be discharged into Lakes Erie and Ontario</p>
<p>Why is it the "United States and canada set limits" and not "the United States and Canada have set limits"?</p>
<p>Wait…AHHH… I’m trying to read this but I’m having a little trouble. The first sentence isn’t even complete. weird…</p>
<p>I feel like this is pretty obvious so I might be missing something or getting this wrong. In order to get the tenses of the sentence to agree, it cannot have “have set”. It is talking about something in the past, nothing something that has just been done/is currently happening/will happen now (hard to explain, but think about it). If it started with “To reduce pollution…” then you would use have set (and it would be “that can be discharged”, but because it begins with “In 1972…” it is talking about something that has already happened.</p>
<p>Usually, when you specify a year like 1972, you are merely talking about an event in the past and use the simple past tense (“set”). The present perfect tense (“have set”) is used to convey a present acknowledgement of that past event. For example, if I say Michael Jackson has sold a large number of albums, I am acknowledging that as of now he sold many albums in the entire past. The present perfect “has sold” accumulates everything he has done in the past (in regard to selling albums) into one acknowledgement statement made in the present. This is different from merely talking about a past event, such as saying He sold out Madison Square Garden in 2 days or He was born in 1958. Alternatively, if you say He has sold out Madison Square Garden in 2 days, you are acknowledging that he holds that particular accomplishment today.</p>
<p>It would be inappropriate to say In 1958, Michael Jackson has been born. It would be correct to say In 1958, Michael Jackson was born. Usually, when there is a year mentioned in the sentence, you use the simple past tense (“was born”), not the present perfect tense (“has been born”).</p>