Saylor.org's English Composition/Subject/Verb Disagreement
When you write a sentence, make sure that your subject and verb agree. For example, do not use the plural first person conjugation of a verb when your subject is the singular first person. "I are" is wrong and it sounds wrong. Now, it's hopefully unlikely that you'd make such an obvious mistake, but there are tricky situations in which you might not be sure which verb form to use that often arise when combining subjects. For example "You/He/She/They and I" will take the plural first person conjugation. The conjunction 'and' in this sentence makes all the difference. If it were "You/He/She/They or I" will simply take the singular first person. "You or I am" not "You or I are".
Also remember that some words take singular third person form when you would think they take plural form, an example being the word 'everyone'. "Everyone knows" is correct, not "Everyone know". On the other hand, remember "All people know" not "All people knows". Just remember the 'one' in 'everyone' is an important distinction. It implies each individual, whereas 'all people' implies a collective whole. Other words like 'everyone' include each, each one, either, neither, everybody, anybody, anyone, nobody, somebody, someone, and no one. All take singular third person form.
The good news is that in English, many verbs take the same form no matter how you conjugate them (I eat, you eat, he/she/it eats, we eat, they eat).