Chances are, if you give your preschooler one cookie, he'll beg you with: "Mommy please can I have more cookies?". He may even ask you for "10 cookies" knowing that he wants a lot of them. Children at this age work hard during their play to understand number concepts such as one-to-one correspondence (counting) and ordinality (small numbers relate to smaller groups of items and larger numbers apply to larger groups). Use situations like this to introduce the concept of plurals. Link language and mathematical concepts with the ideas that one item is singular and many items (or more than one) are plural.
Show your child one block from a pile of identically colored and shaped blocks and ask him "what is this?". He may answer "a block" or "block." Then say "yes a block, I have one block."
Set out a pile of four or more blocks and ask you child "what is this?". He may say "blocks" or "lots of blocks." Respond with "yes, I have more than one block. I have many blocks."
Count how many blocks you have together, then say "when you have more than one, you have a plural amount."
Show your child one picture card and ask "how many ducks are on this card?". If there is only one duck, explain that this card has a single duck. Next, show him a card with many ducks and ask "how many ducks are on this card?". When he answers, respond "When you have more than one duck, we say ducks because it's plural. We have three ducks on this card."
Ask your child to sort the cards into piles of singles and plurals. Comment positively as he works, with statements like "good work, I see you put the card with two horses in the plural pile and the card with one pig in the single pile."
Point out singles and plurals in everyday play and interactions. If your child is playing with two dog toys, say" how many dogs do you have? Are there more than one?". Daily conversations about one item versus many items and modeling the proper use of plural language will reinforce the concept of plurals for your preschooler.