Label the subject or subjects and verb in each sentence.
when i was mowing the lawn, a little frog hopped out of the grass.

In the sentence above, "I" and "frog" are the subjects, while "mowing" and "hopped" are the verbs in the sentence.

Explanation: In the sentence above, "I" and "Frog" are the doer of the action, therefore they are the subjects. Also, the action been performed are "mowing" and "hopped".