Choose the tenses of your verbs accurately to express the timing or sequence of events that you are describing. Often, the particular sequence of events that you are describing will require you to use several different verb tenses within a single sentence or paragraph. Although it is appropriate to vary your verb tenses in accordance with the actual timing of the events, you should avoid shifting tenses unnecessarily.
Pay special attention to how you express the sequence of tenses when describing the timing of events or when paraphrasing ideas.
To emphasize that an event occurred or was completed before another event, use a form of the perfect auxiliary have.
By 150 B.C., the Greeks had accurately
determined the shape and dimensions of the Earth and the
distance of the Moon, but had not managed
to probe very far beyond that.
--Isaac Asimov, The Universe
The reaction appears to have finished already.
To express that an event in a participial phrase occurred at the same time as the event described in the main clause, use a present participle.
Arising from the complex flow and interaction of
information, life is a dynamic process.
--Kai Wu, "Artificial Life," SciTech Magazine (modified)
To emphasize that an event in a participial phrase occurred before the event described in the main clause, use a perfect auxiliary and a past participle.
Having examined the peculiar data carefully, the
investigators concluded that the equipment was faulty.
When writing formal documents, use the simple past tense to describe habitual actions. Although the modal would is often used to express past tense habitual actions, its use is generally restricted to spoken and informal contexts.
Before the development of anesthesia techniques, surgeons would
prepare their patients for surgery by getting them drunk.
Before the development of anesthesia techniques, surgeons
prepared their patients for surgery by getting them
drunk.
When paraphrasing a written idea, introduce the paraphrase with a present tense verb (e.g., writes) and do not change the original verb tenses of the idea.
In his book The Universe, Isaac Asimov seeks
to trace the steps by which mankind's grasp of the universe
widened and deepened.
When paraphrasing an ongoing or future spoken idea, introduce the paraphrase with a present or future tense verb (e.g., says) and do not change the original verb tenses of the idea.
At tomorrow's meeting, she will officially
announce she will be chosen yesterday to
lead the government research project.
At tomorrow's meeting, she will officially
announce she was chosen yesterday to lead
the government research project.
When paraphrasing a previously spoken idea, introduce the paraphrase with a past tense verb (e.g., said) and change the verb tenses of the idea to match the tense of that verb.
The engineer informed us that she is not
able to field-test the device yesterday because it is
raining.
The engineer informed us that she was not
able to field-test the device yesterday because it was
raining.
If a previously spoken idea is a general fact that is always true or concerns a future event that has not yet occurred, you can either maintain the original verb tenses of the idea or shift them to past tense to match the tense of the verb you used to introduce the idea. Maintaining the original tense emphasizes the continuing validity of the idea, whereas shifting the verb tenses emphasizes the narrative quality of the paraphrase.
Von Neumann postulated that an automaton will need at
least six elements: a computer, a sensor, a manipulator, a cutter, a fuser, and
"girders" from which the automaton will be built and which
encodes instructions to act as "memory."
[continuing
validity emphasized]
Von Neumann postulated that an automaton would need at
least six elements: a computer, a sensor, a manipulator, a cutter, a fuser, and
"girders" from which the automaton would be built and which
would encode instructions to act as "memory."
[narrative quality emphasized]
--Kai Wu, "Artificial Life," SciTech Magazine (modified)