The second sentence usually has a prompt.
Example
The following is an example of a transformation question:
Call me immediately he arrives Call me __________ he arrives (soon)
Answer: Call me as soon as he arrives
In the classroom
Transformation exercises involve learners in consciously manipulating language patterns, and can raise their awareness of structure. They are a common test item, where they are used to test discrete items of language, usually structural.
Further links:
https://www.teachingenglish.org.uk/article/test-question-types
https://www.teachingenglish.org.uk/article/planning-a-grammar-lesson