In the following text, should I replace the highlighted can’t with couldn’t?
Furthermore, the third point the lecture uses to contradict the passage is that organic foods don’t benefit small famers. While the reading claims that organic foods allow small farmers to make high profits from their business, the professor in the lecture completely denies that idea. To support his belief he says that since 1982 small farmers can’t make a living off of producing organic foods. That is because big companies always buy their foods; and the professor encourages us to buy any organic food and attempt to see the producer. He is sure that we won’t see any small farmer’s name. Consequently small farmers will continue to face economic problems in the future if they depend only on organic foods.
A time expression headed by since defines a timeframe which extends from a point in the past up to the ‘Reference Time’, the time you are talking about. A time frame with this particular shape usually calls for a perfect construction. Since your paragraph is cast in the present tense, you are talking about the present, and you should employ a present perfect:
The professor says that since 1982 small farmers have not been able to make a living off of producing organic fooods.
Source : Link , Question Author : Marco Dinatsoli , Answer Author : StoneyB on hiatus