Final answer:
To combine the sentences using 'but', we connect two contrasting independent clauses with a comma before 'but'. The correctly combined sentences are: 'The children did not have to go to school, but they had a hard life.' and 'The nights in the desert are cold, but the days in the desert are very hot.'
Step-by-step explanation:
To combine the given sentences using the coordinating conjunction 'but', we should remember that 'but' is used to connect two independent clauses that present contrasting or opposing ideas. In each case, the sentences before and after 'but' can stand alone as separate sentences, so we need to ensure we have correct punctuation when joining them.
The combined sentences using 'but' would be:
- The children did not have to go to school, but they had a hard life.
- The nights in the desert are cold, but the days in the desert are very hot.
In these compound sentences, the use of the comma before 'but' is necessary because it precedes an independent clause. This complies with the rule of creating a compound sentence using a coordinating conjunction.