Subject Verb Agreement
When writing, it's important to make sure that your subjects and verbs 'agree' with each other. That means that plural subjects should be matched to plural verbs and singular verbs should be matched to singular subjects. If you don't do this, the sentence is not only grammatically incorrect but confusing to boot.
Singular and Plural Subjects
Present Tense
Let's look at subject-verb agreement in the present tense.
The valet crash the car. The owners is upset.
Both of those sentences probably look and sound funny to you, but let's examine why. In the first sentence, we have a singular subject - that's 'the valet' - and in the second sentence, we have a plural subject - that's 'owners.' Quick refresher: to find the subject, first find the verb, and then find the thing the verb is connected to - usually the noun to left of the verb but not always. In the present tense, to make a verb singular, you usually add an '-s' or '-es', while to make a noun singular you remove the '-s'.
So, to make the subjects and verbs agree in the first sentence, we would either have to change the subject to make it plural (since the verb is plural) like so - The valets crash the car. or, more sensibly, change the verb to make it singular by adding '-es', making it - The valet crashes the car. Likewise, in the second sentence, you must change the form of the verb 'to be' to its plural form or change the subject to its singular form, making it either - The owner is upset. or The owners are upset.
To remember where to put your plurals, think of subject-verb agreement in the present tense as a little bit like a see-saw: When the subject is plural, it gets the plural form (usually an an '-s'), and when the subject is singular, the plural drops off the verb and gets the '-s' or '-es' this time.
Past Tense
In the past tense, however, subject-verb agreement is a little less complicated. Why? Because in the past tense, regardless of whether your subject is singular or plural, the verb form stays the same.
So, The robot menaced me from across the bar. and The robots menaced me from across the bar. keep the same verb form, though the first sentence has a singular subject ('robot') and the second sentence has a plural subject ('robots').
Of course, there are some exceptions! (This is English; there are always exceptions.) When certain helping verbs are used as the main verb, then you have to pay attention to your subject-verb agreement again. The most common are 'has'/'have' and 'was'/'were.' Take this example:
The robot has destroyed the village. Both 'has' and 'robot' are singular, and 'has' is the main verb here. If the main subject is plural, then you must also pluralize 'have', as in, The robots have destroyed the village.
Or, The photo was plastered all over campus. compared to The photos were plastered all over campus. 'Photo' - 'was.' 'Photos' - 'were.'