13a Brent studies hard before he goes to bed.
He agrees with Brent to the extent that it is a masculine singular form. Its being in the subjective case is a kind of agreement with its function in the sentence. Similarly goes agrees with he just as studies agrees with Brent, for both have the "s" ending which occurs when the subject of a verb in the present tense is not I, you, we, they or their equivalents. Such agreements are second nature for a native speaker, who would never say
13b Brent study hard before him go to bed,
which nevertheless is a lot easier to interpret than, for example,
13c Hard before Brent studies to bed goes he,
where all the right agreements could be said to occur. The errors once again illustrate the importance of word order in English.
Getting the agreements right ought, in theory, to be relatively easy for native speakers, since their instincts constitute grammar (meaning 1). Perhaps if those with a professional interest in correct grammar let things evolve naturally, we would have no difficulties in this respect. But our language instincts do produce some apparent anomalies. For example, all children, left to their own devices, will say
14a Me and Darby went to the store,
though none but an infant or Tarzan would say
14b Me went to the store.
So we are taught, from a tender age, that "me and Darby" is egocentric and ungrammatical. Unfortunately people who have been taught to say
14c 3 Darby and I went to the store
are likely to say, when they grow up,
15a It occurred to Darby and I that going to the store was a poor idea,
though they would never say
15b It occurred to I that going to the store was a poor idea.
In short, agreement is a muddling sort of business however we go at it. The only way to get it right consistently is to learn how to analyse the structures of your speech, which is to say, how you put sentences together.
See also the pages on the subjective case and the object case.