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I can't think of an instance where I would ever say "shoulders are the target". If you are talking about shooting, I would say "aim for the shoulders".
I could imagine saying:
1: My shoulders are the source of the pain.
This is a similar construct to your example in that we have a plural subject linked to a singular predicate noun, and the verb agrees in number with the subject. But if we reversed the two, I would say:
2: The source of the pain is my shoulders.
It is possible for a seemingly plural subject to take a singular verb:
3: Scalloped potatoes is an excellent side dish for any dinner.
This works because "scalloped potatoes" is being considered as a single entity here. It's a bit complicated, but please check out this thread for more insight:
I hope that this has been helpful. I would also like to address some presentation points, if I may.
First, you will see that I have changed your subject heading. We often need to refer back to earlier threads, as I did with "Spaghetti and meatballs ... " above, so it helps if the subject heading gives some information. Your original heading, "Which one is correct", was too vague.
Also, when you are speaking about a certain word or phrase in another context, it should be identified as such, either with boldface, highlighting, italics, or just quotation marks, as I did with "scalloped potatoes" above. This makes your question much more readable and less confusing.
Finally, please avoid informal abbreviations on this forum. We are here to help you improve your grammar, and it's sometimes difficult for us to distinguish between shorthand and an actual mistake. There is no such word as "wanna".
So, you wrote:
I just wanna make sure.
"Shoulders are the target." Is this correct? Or is the target or, i dont know, somehow i have doubts evn though it is easy. I just hear ppl using is even when plural, and you cannot always explain it with an existed rule. Thank you.
I would revise this as follows in accordance with my second and third points above, ignoring any other stylistic objections I might have:
I just want to make sure.
"Shoulders are the target." Is this correct? Or "is the target", or, i don't know, somehow i have doubts even though it is easy. I just hear people using "is" even when plural, and you cannot always explain it with an existed rule. Thank you.
I think this is closer to what you meant to say. The word "existed" doesn't work here, but that's beyond the scope of the point I wished to make.
If you don't mind my asking, is English your native language, or are you from somewhere else?