Is the following sentence correct?

"This limits the algorithm to relationships the user already knows exist."

As a non-native English speaker I am struggling to determine which verbs should be plural and which should be singular.

Where I am having difficulty is with the "exist(s)" and "know(s)". How I argued was that "relationships" is plural so I put it with plural "exist" and the "user" is singular so it goes with singular "knows".

But I'm not sure weather this is right or weather I must apply the "user" singular-ness to both "knows exists" because it is closer to it? I am really confusing myself.

(I know starting the sentence with "this" seems confusing but in context of the thesis it is part of it makes sense)

  • If it were "users" it would be "know". With the verb "limits" we can either use a plural or a singular noun here. This limits the algorithm or This limits the algorithms. If it were "exists" instead of "exist" it would be "This limits the algorithm to a relationship the user already knows exists." Mar 31, 2017 at 10:35

2 Answers 2


This limits the algorithm to relationships the user already knows ___ exist.

"Limits" is singular because of singular pro-form "this", which is anaphoric to some syntactically singular expression presumably mentioned elsewhere.

"Knows" is singular because it is governed by the singular NP subject of the relative clause ("the user already knows exist").

The covert relativised element represented by gap ( ___) is subject of the embedded "exist" clause. Gap is anaphoric to the plural noun "relationships", and hence a plural verb-form is required. We understand that "the user already knows (that) relationships exist".

  • "This limits the algorithm to relationships the user already knows exist."

  1. This is an unknown thing?
  2. "limits" is a verb here that means "reduces in quantity or extent".
  3. "algorithm" is singular.
  4. "to relationships" is plural (to a certain extent).
  5. "the user" is some particular user (we know who)
  6. "already" by this time.
  7. "knows" to be aware of, be certain of.
  8. "exist" plural form.

  • Something narrows the algorithm to known relationships that already exist and the user knows about.

knows refers to the user while exist refers to relationships.

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