I originally did this list as an Instagram post, but then it occurred to me that I should put it up here as well, so here goes in slightly expanded form.
First, remember that the singular/plural rule for verbs is the opposite of the rule for nouns:
Third-person singular verbs end in -s (it works, s/he does, the graph shows).
Third-person plural verbs do not end in -s (they work, they do, the graphs show).
1) Compound subject = plural
A compound noun consists of two nouns joined by and. These subjects are always plural, regardless of whether the individual nouns are singular or plural. This rule is easy in principle but can be surprisingly difficult in practice.
Correct: A stressful atmosphere and poor management are often responsible for employee burnout.
Incorrect: A stressful atmosphere and poor management is often responsible for employee burnout.
2) Prepositional phrase between subject and verb
A prepositional phrase begins with a preposition (e.g., to, of, in, on, with, by, for, from) and ends right before the verb.
Correct: A lack of access to fresh fruits and vegetables contributes to many people’s difficulty losing weight.
Incorrect: A lack of access to fresh fruits and vegetables contribute to many people’s difficulty losing weight.
3) Non-essential information (word, phrase, or clause) between subject and verb.
Non-essential information is placed between two commas and can be removed from a sentence without affecting its basic structure or meaning.
Correct: Fresh fruits and vegetables, which are difficult to find in some neighborhoods, are an important part of a healthy diet.
Incorrect: Fresh fruits and vegetables, which are difficult to find in some neighborhoods, is an important part of a healthy diet.
4) Gerunds (-ing forms) are always singular
In its simplest form, this rule is generally easy to apply because errors sound so obviously wrong.
Correct: Skiing is one of my favorite activities.
Incorrect: Skiing are one of my favorite activities.
But it can get very challenging when the gerund begins a longer complete subject that contains a plural noun or nouns.
Correct: Spending many hours sitting in front of a computer each day makes it difficult for people to lose weight.
Incorrect: Spending many hours sitting in front of computer each day make it difficult for people to lose weight.
5) The verb after that or who agrees with the noun before
Native English speakers generally don’t have problems with this one, but it’s a major issue in IELTS essays.
Correct: People who spend many hours sitting at work each day often find it difficult to lose weight.
Incorrect: People who spends many hours sitting at work each day often find it difficult to lose weight.