See if you can find the problems ? There is only one problem in each sentence.
1: Johnson (1) has (2) no equal as a salesman; his commissions are (3) larger than those made by almost any other (4) salesman.
2: (1) If people (2) had helped solved the energy crisis,(3) it might (4) of have been a lesson to humanity.
3: The student (1) thought that (2) the lecturer had (3) referred (4) to The Communist Manifesto.
"The Communist Manifesto" should be underlined.
4: He (1) has always been negligent (2) about his health, and he (3) is feeling the (4) effects of his carelessness.
I see nothing wrong with sentence 1 or sentence 4. There are many stylistic improvements that could be made, but everything appears grammatical.