Ever since you were very little, you have known, or at least felt, that the subject of a sentence needs to agree with its predicate — with its verb, really — in person and number. This is not a very hard problem, since most English verbs change only in the third person singular. For example, “to have” is “have” for “I,” “you” (singular and plural), “we,” and “they.” It becomes “has” only for “he,” “she” and “it.” In the past tense, it’s all “had.” That is not very hard at all.
The verb “to be” takes it up a notch, as it changes more often: “are” for “you” (singular and plural) as well as “we” and “they“; “am” for “I,” and “is” for “he,” “she” and “it.” In the past, it is “was” in the singular and “were” in the plural. Every once in a while you will hear someone say something like “How you is?” or “I is the boss.” Even so, there’s no risk of mistaking this for correct English.
How would you evaluate this one? “Jimmy and I are leaving now.” Is “… I are …” correct? If so, why?
What about this? “Either Ruth or I am going to do the cooking.” Is “ … am … ” appropriate with “Ruth or I“? (Many would say, “Either me or Ruth are going to do the cooking,” and that is really wrong — so wrong that it makes you feel bad for the utterer.)
Here’s the story on verb agreement when you have more than one subject (technically, we call them “compound subjects“):
First: Remember that the order of persons in a series is (1) third person, then (2) second person, then (3) first person. Example: “Your brother, you and I will be riding together.” If you put the first person or the second person in the wrong place, it can mislead you to use the wrong form of the verb. Plus, it’s wrong.
Second: When you have a conjoint compound subject — meaning that it’s inclusive (using “and“) rather than exclusive (using “or“), the subject must, obviously, be plural. Why? Because there’s more than one actor. Thus, you must use a plural verb form. In English, the plural forms are the same for first, second and third person, so there’s no agony of choice here. “Plural” is all you need to know. Examples: “Maggie, Millie, Molly and May went down to the sea to play one day” (all third person); “Maggie, Millie, Molly and I went down to the sea to play” (third and first); “Maggie, Millie, Molly and you went down to the sea to play” (third and second). It’s all the same, and it does not matter whether you are using the present tense, the past tense, the future tense, or a perfect tense. Conjoint compound subjects are easy: the verb is always in the plural.
Third: When you have a disjoint compound subject, that is, one that uses “or,” the verb must agree with the last subject mentioned. Therefore, “Either Ruth or I am going to do the cooking” is correct. So also is: “Green beans or spinach is what we will make.” Conversely, this is correct, too: “Spinach or green beans are what we will make.”
That’s all there is to this subject. Really, it is easy until you get to point three, and then you will have to do a little thinking. But that’s OK; thinking is good for you: It makes you smarter.