Question 1:

Error: The conditional sentence is incorrect. The correct form is "If Lucia were here now, she would find out..."
Corrected sentence: If Lucia were here now, she would find out the truth about her uncle's accident.
Question 2:

Error: The use of "doing" is incorrect. It should be "to do."
Corrected sentence: If a student takes a course on Computer Science, it will take him four years to do the course.
Question 3:

Error: The double negative "did not rain" is incorrect. It should be "rained."
Corrected sentence: Unless it rained, Peter would pay us a visit.
Question 6:

Error: The verb tense is incorrect. It should be "could go home."
Corrected sentence: If she had finished the work, she could go home.
Question 13:

Error: The verb tense is incorrect. It should be "will be sacked."
Corrected sentence: Unless you work harder, you will be sacked because of your laziness.
Question 16:

Error: The verb tense is incorrect. It should be "reduce."
Corrected sentence: If we reduce the speed of population growth, there will be less pressure on the earth.
Question 18:

Error: The verb tense is incorrect. It should be "would soon have to face."
Corrected sentence: If we continue to use fuels at the current rate, we would soon have to face a fuel crisis.
Question 19:

Error: The verb tense is incorrect. It should be "would run out of."
Corrected sentence: If she bought that house now, she would run out of money.
Question 20:

Error: The verb tense is incorrect. It should be "would you do."
Corrected sentence: What would you do if you could speak French well?
Question 35:

Error: The verb tense is incorrect. It should be "would go."
Corrected sentence: If I were you, I would go to the dentist's and have the tooth checked.