Relative Pronouns

When covering the lesson on relative pronouns, participants often forgot to agree the subject and verb and did not add the 's' ending in third person singular. Sentences they constructed looked like this: A pilot is someone who fly a plane. – instead of the correct: A pilot is someone who flies a plane. And since this mistake was made even by quite advanced participants, I would like to remind you how important subject-verb agreement is in the English language.