The perfect and progressive auxiliaries combine with the present or past tense, or with modals, to form complex verb phrases with special meanings (32f).

Distinguishing the simple present and the present perfect

My sister drives a bus.

The simple present (drives) tells us about the sister’s current job. But if you were to add the phrase for three years, it would be incorrect to say My sister drives a bus for three years. Instead, you need a time frame that goes from the past up to the present. The present perfect or present perfect progressive expresses this time frame.

My sister has driven a bus for three years.

My sister has been driving a bus for three years.

Distinguishing the simple past and the present perfect

Since she started working, she has bought a new car and a DVD player.

The clause introduced by since sets up a time frame that runs from past to present and requires the present perfect (has bought) in the subsequent clause. Furthermore, the sentence does not say exactly when she bought the car or the DVD player, and that indefiniteness also calls for the perfect. It would be less correct to say Since she started working, she bought a new car and a DVD player. But if you say when she bought the car, you should use the simple past tense.

She bought the car two years ago.

It would be incorrect to say She has bought the car two years ago because the perfect cannot be used with definite expressions of time. In this case, use the simple past (bought).

Distinguishing the simple present and the present progressive

Use the present progressive tense when an action is in progress right now. In contrast, use the simple present for actions that frequently occur during a period of time that might include the present moment (though the simple present does not necessarily indicate that the action is taking place now).

My sister drives a bus, but she is taking a vacation now.

My sister drives a bus, but she takes a vacation every year.

Many languages use the simple present (drives, takes) for both types of sentences. In English, however, the first sentence would be incorrect if it said but she takes a vacationnow.

Distinguishing the simple past and the past progressive

Sally spent the summer in Ecuador.

The simple past tense is used in this case because the action occurred in the past and is now finished.

The past progressive tense is used relatively infrequently in English. It is used to focus on duration or continuousness and especially to call attention to past action that went on at the same time as something else.

Sally was spending the summer in Ecuador when she met her future husband.