Explanation:

- From the given information, 'they were not playing' is coded as 'su to al zi'.

- 'they are singing tomorrow' is coded as 'to ej di mo'.

- 'are you playing' is coded as 'su mo pa'.

- 'you were sleeping' is coded as 'pa zi qu'.

- Analyze the repeated and unique words.

- ‘they’ is common in the first and second sentences. Matching code: 'to'.

- ‘were’ is common in the first and fourth sentences. Matching code: 'zi'.

- ‘not’ appears only in the first sentence. In its code, only 'al' is unique.

- Based on elimination:

- Correct Answer for 'not': al