37.He got a bad mark because he was lazy. (because of)

 He got a bad mark because of his laziness.

38. The content of the film wasn't good. I enjoyed the acting. (Although)

 Although the content of the film wasn't good, I enjoyed the acting.

39 We usually walk to school. (on) 

  We usually go/travel/get to school on foot.

40. That is his invention. (Possessive pronouns)

  That invention is his.