Consider first the following sentences :
The guy seems kind of cute.
The guys seems kind of cute.
The verb has-s added whenever the subject is third person singular. Such a relationship is called agreement or subject-verb agreement.
Now consider these sentences :
The guy we met at the party next door seems kind of cute.
The guys we met at the party next door seem kind of cute.
The verb seem must agree with the subject, The guy or The guys, and that agreement takes place over a long distance. In the example above, the distance encompassed is we met at the party next door, but there is no limit to how many words may intervene, as the following sentence illustrates :
The guys (guy) we met at the party next dorr that lasted until 3a.m and was finally broken up by the cops who were called by the neighbours seem (seems) kind of cute.
This aspect of linguistic competence is explained by the phrase structure tree of such a sentence, which is shown below omitting much detail :
In the tree, ' ====== ' represents the intervening structure, which may, in principle, be indefinitely long and complex. But speakers of English know that agreement depends on sentence structure, not the linear order of words. Agreement is between the subject, structurally defined as the NP immediately below the S, and the main verb, structurally defined as the verb in the VP immediately below the S. Other material can be ignored as far as the rule of agreement is concerned, although in actual performance, if the distance is too great, the speaker may forget what the head noun was.
