In any sentence or clause, the subject must agree with the verb, e.g. 'The boys play football on Saturdays, but John plays only on Friday'. The subject 'boys' is plural, so the verb is required to be in the plural form 'play'. The subject 'John' is singular, so the verb is required to be in the singular form 'plays'.
When you hear the English language, you will probably be able to detect a wrong subject/verb agreement, even if you don't know that's what it is. A word or words will sound wrong. Occasionally, in written language, it's harder to detect as our brains tend to read what should be correct. This is all very nice, and I am sure that you don't have any problems with this stuff. Unfortunately, there are situations that can cause some difficulties.
This 11-plus quiz is a bit tricky, so take your time and read the questions carefully, even saying them out loud if it helps.