Student or Learner
To a student, how would one introduce the topic of subject and verb agreement. What will be cover and touch before jumping to its rules..?
This can be done with little children, and pictures of one animal, or more than one.
Here is one dog.
Here are two dogs.
<Show a picture>
Teacher: How many dogs are there? (ask them to use a full sentence to reply).
Learner: There is one dog / There are three dogs. (Rephrase an incorrect answer)
Teacher: What is happening in this picture?
Learner: The dog is running. The dogs are running. (Rephrase an incorrect answer)
Once they understand that 'is' is for one, and 'are' is for more than one, you're halfway home.