I wrote this sentence:

We suppose that two items are similar if they have been bought together by many people.

Should I say?

We suppose that two items are similar if they were bought together by many people

That sentence is the premise of a theory, it is a fact, not opinion.

item means product in this context

  • @Cardinal like we assume that two products are similar if many users bought them together. got me please ? – Marco Dinatsoli Aug 6 '16 at 21:31
  • @Cardinal that is a theory we are trying to check. the thing is not in the meaning of the sentence but in the grammar, don't worry about the truth (validity) of the sentence, that is what we are proving – Marco Dinatsoli Aug 6 '16 at 21:32
  • @Cardinal great, thanks man, is it clearer now ? – Marco Dinatsoli Aug 6 '16 at 21:38
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    @AlanCarmack I do not see any conditional sentences on that link. – Cardinal Aug 6 '16 at 22:03
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    @MarcoDinatsoli - Please read this link and decide whether you need to use the perfect here. Note the bolded advice: "Don't use the perfect unless you need it." Or try FumbleFingers's answer. Either one will help you. – P. E. Dant Reinstate Monica Aug 7 '16 at 1:11

We suppose that two items are similar if they were* bought together by many people.

You only 'suppose' if you have evidence to support a supposition. So when put together, these bolded statements make sense. 'If' is the modifier here. Your supposition only exists because of the word 'if.'

*You use 'were' in more affirmative statements. It it like subjunctive, but not quite - since English has no subjunctive, truly. It only has it in some cases. It is suitable for proving theory, since it implies you know that if products are bought together, they [then/therefore] are similar.

  • You use 'have been' in more present situations. If things are being bought right now, right here, in this moment, then it is appropriate to use 'have been.' It's a more hypothetical case. It means you only suppose your supposition if the following (buying products) has occurred. If it hasn't, then you don't suppose anything. You continue on supposing that they're dissimilar, until they are bought together.
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