Some questions in English can be answered with YES or NO:
- Are you Spanish? - Yes/No

We can also give a long answer to it:
- Are you Spanish? - Yes, I am Spanish
- Do you like going to the cinema at the weekend? - No, I don't like going to the cinema at the weekend

Answers with Yes or No are too short and sometimes sound a bit rude. Long answers are too long and repetitive. But there is something in the middle: we call it "Short answers" and they are short, but more polite than saying simply Yes or No. That is why short answers are very common in English:
- Are you Spanish? - Yes, I am.
- Do you like going to the cinema at the weekend? - Yes, I do.

RULES

1- A short answer has three words: Yes/No, + Subject + Verb
- Can you remember? - Yes, I can
- Will you come here? - No, I won't

2- The verb at the end of the short answer is the verb at the beginning of the question.
- Have you been to London before? - Yes, I have
- Do you think she likes me? - No, she doesn't
- Can you cook? - No, I can't
- Does she live here? - Yes, she does

3- The short answer always have 3 words, so we can use contractions to adjust the total number of words.
- Are you French? - No, I'm not    (not: No, I am not = 4 words)
- Are you French? - Yes, I am    (not: Yes, I'm = 2 words)