Exercise 7. Use Present or Past Indefinite:
A.
A dolphin (to look) like a big fish with a built-in smile. Actually dolphins (to belong) to the same group of living creatures that humans do: mammals. They (to grow) larger than we do — up to ten feet long. They (to live) in water but must come to the surface to breathe. And though humans (to consider) themselves the most intelligent of all mammals, dolphins may not be too far behind. A measure of a mammal’s intelligence (to be) the ability to communicate. Dolphins (not to use) words in our sense, of course. Dolphin talk (to be) a variety of whistles, clicks and creaking sounds. But it (to work). In one experiment, Dolphin A was able to tell Dolphin B which level to press if he (to want) a reward of fish. Only Dolphin A (to know) the secret. There (to be) no way for Dolphin B to find out unless Dolphin A (to tell) him.
We (not to know) if dolphins (to call) one another by name as people do, but something similar (to seem) to be true. Each dolphin (to have) a whistling signal, different in tone and pattern from the signals of other dolphins. When a baby (to be) born, its mother (to whistle) almost constantly for several days. The baby (to learn) to know its mother’s voice pattern, and (to come) when she (to call). Dolphins (to imitate) words, laughter and even the tones of different voices. Dr. John Lilly, a dolphin researcher, once (to displease) a dolphin he was working with. The dolphin (to scold) him angrily in his own voice and (to copy) Dr. Lilly very clearly.
Humans (to think) that they (to be) superior to dolphins. But what dolphins (to think) about humans? (After “Dolphins” by A.H. Lehrer)

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