If you ask a student: “When you were in Paris, did you see the Eiffel Tower?” Obviously, the student can answer “Yes”, or “No”. Either of these answers could be considered to be correct. If, as a teacher, we were to ask you: “Is this a good way to teach ESL students?” the answer would have to be “No”.
Any beginner ESL student could be taught to say “Yes” or “No” at the appropriate time, without having the faintest idea what the question is about.
On the other hand, let’s look at what the student can learn if they answer: “Yes, when I was in Paris I saw the Eiffel Tower.”
By giving a complete sentence, the student learns the proper usage of the vocabulary words. The student can use most of the question in the answer, but it requires that they change the person from “you” to “I”. This also means switching the verb form. Practice with English language word order is also given. If the students repeat English sentences often enough, our English word order will start to come naturally to them. You can teach your students that if the question starts with “Did…” to always answer in the past tense. Finally, requiring a sentence answer requires careful listening to the question, so the students increase their listening comprehension skills.
For these reasons we usually say: “Answer in sentences.” at the start of all the written exercises in our ESL Curriculum.
Occasionally at the start of the school year, I would encounter a student who gave a one word answer. I would explain that the students had paid a lot of money for their course, and that they had gone to a lot of trouble to get to class that day, so I didn’t want them to miss the opportunity to really practice their English, so I needed a better answer, a complete sentence.