OK, what are we talking about here?
First you need to be able to find verbs in a sentence, and there will be one at least 99.9% of the time. Then you ask, WHO? WHAT? + VERB. Congratulations: you have found the SUBJECT!
You see, all those verbs and subjects agree. You wouldn’t say I is joking or Chinese students works hard, would you?
So why do people still forget this?
If you have as your first language one that does not mark subjects and verbs by changing something, like Chinese or Indonesian or Vietnamese, then it is easy to forget about it in English. After all, it doesn’t really make much difference to the meaning of what you say; it just sounds, well, silly to an English speaker — like some poor English speaker trying to cope with tone in Mandarin and getting it wrong, except actually less serious than that. (I once introduced myself to a group of Chinese in my very poor Mandarin thus: “Hello. I am Neil Whitfield and I am a dumpling.” when I really meant “teacher.” Tone, you see…)
It is important not to get so nervous about agreement that you slow your writing down to a crawl. And for heaven’s sake, throw away the white-out! Better to get your ideas down quickly and correct for agreement later, by crossing out if necessary.
The good news is that it is not such a problem when you write in past tense, as English doesn’t really change much except in present tenses. The bad news is most essays are actually written in present tense, except in History and in some other types of Factual Report writing. That’s because talking about literature, or stating general truths, is done in present tense — just like this sentence, really!
Be glad if you learn some French or German or Latin, or any other European language. These studies make you much more aware of subject-verb agreement because many other European languages mark agreement much more than English does.
Even native speakers of English have trouble with subject-verb agreement under certain circumstances; in a long sentence, for example, you may have a number of words in between the subject and the verb: it is easy to lose track and forget to make the subject and verb agree. And there are some words, especially ones that stand for groups (like ‘the team’) where your choice is not easy. Should it be the team is or the team are? Well, that depends on whether you are thinking of “the team” as a single unit (is) or as many individuals (are).