Answer:
We need to reach the airport on time lest our plane may take off without us.
Step-by-step explanation:
Subordinating conjunctions of condition are those clauses that are used to specify the conditions of why such and such must happen or is happening. In short, they describe the condition in the sentence.
In the given example, two sentences must be joined by subordinating conjunction of condition to make a new sentence. This means that the subordinating conjunction of condition will join the two sentences and make it in a single sentence, providing a suitable conditional conjunction.
The new sentence will be
We need to reach the airport on time lest our plane may take off without us.
In the sentence, "lest" is the subordinating conditional conjunction that provides the condition for why we must reach the airport on time.