A conditional sentence has two connected parts.
● Condition (شرط): the event introduced by إِنْ.
● Answer of the condition / consequence (جوابِ شرط یا جزاء): what follows if the condition occurs.
In إِنْ تَجْلِسْ أَجْلِسْ:
Condition: تَجْلِسْ — you sit.
Answer: أَجْلِسْ — I sit.
The second action is understood in relation to the first.