You will need: objects in groups of one or more (eggs, apples, boxes, cats, flowers, umbrellas etc - try for a mix of beginning with vowels and consonants) pens and paper.
Put all the objects on the rug - 'Wow, I have a lot of different things, can you help me sort them into groups.'
Once sorted ask the child 'can you hand me the butterfly'
ask 'what did you give me?' and as they say the words write it on a strip of paper
Then 'can you hand me an egg' and continue as above with remaining groups.
Pointing to the articles ask 'can you remember what these little words are?' (show light blue pyramid/triangle if needed)
'When you gave me the butterfly there was only one. You knew which one I wanted. When you gave me an egg there were many to choose from so you weren't sure which. You could pick any that you wanted.'
We call 'the' a definite article. We use it with just one, the definite one that I wanted, the definite butterfly.
When you picked one of the many eggs or cars, you had a choice of which to pick; an egg, a car - a and an are indefinite articles.
The word definite comes from the Latin definire which means to be limited.
The definite article limits you to one object or idea.
indefinite comes from the Latin indefinire which means to not be limited.
The indefinite article does not limit you to one object or idea.