There are many different ways singular and plural can be introduced to second grade students. It would be beneficial to know if you were talking about nouns or verbs, but both can be addressed.
One wonderful way to introduce the concept of singular and plural is to use cross-curricular ideas. Given that they have already studied addition facts, they are already familiar with singular and plural. Therefore, to introduce singular and plural (in regards to nouns) use concrete objects. For example, you could use animal figures (a staple in the elementary classroom) to have them identify a singular noun (one) and add to the initial animal to introduce plural.
When discussing verbs, a much harder concept, I have always used the "s" rule. Singular nouns do not have an "s";therefore, the verb that they carry does. Plural nouns have an "s," but their verbs do not.