Echo questions are used in conversation to show interest in what someone is saying by repeating their statement in the form of a question. They do not seek information but rather express interest, concern, surprise, or other reactions depending on intonation. Similarly, "reply questions" are short questions containing an auxiliary verb and pronoun that are used to acknowledge a statement and show the listener is paying attention. The document also provides examples of using "so do I" and "neither do I" to agree or disagree with positive and negative statements in different tenses.