The Korova milkbar sold milk-plus, milk plus vellocet or synthemesc or drencrom, which is what we were drinking. This would sharpen you up and make you ready for a bit of the old ultraviolence.
There are actually two different uses going on here. I think the "would" in "This would sharpen you up" is used to transform the statement into a conditional or hypothetical. For example, "That painting would look nice over the fireplace." In this case the implied condition under which "sharpening up" will occur is the drinking the milk-plus.
In the Bundy examples, "would" is preferred because they are generalized statements about his tactics rather than a recital of his crimes. The same generalization is accomplished without the use of "would" by starting a sentence with "in many cases," although the meaning of the sentence would not significantly change had the author chosen to include the word "would".
The last two examples are perfectly valid constructions either way, but you might prefer one over the other depending on context. The meanings are only very subtly different.
The Aztecs used shells and jewels to create dyes.
The above sentence simply states a historical fact about what the Aztecs did. It works well in multiple contexts.
Example A: "I walked through the exhibit. In one diorama, the Aztecs sacrificed a human being. In another, the Aztecs used shells and jewels to create dyes."
Example B1: "Archaeological evidence suggests that the Aztecs used shells and jewels to create dyes for application to clothing, ceramics, and architecture."
Example B2: "Archaeological evidence suggests that the Aztecs would use shells and jewels to create dyes for application to clothing, ceramics, and architecture."
Note that in Example A, "would use" would not be appropriate. However, in Example B, "used" and "would use" are interchangeable without losing the essential meaning.
The Aztecs would use shells and jewels to create dyes.
The above sentence states the same historical fact, but generalizes it as a habitual aspect in past time. As an example, it might be used when describing a scene or trying build a sense of atmosphere. The "would" might also be used for emphasis or contrast.
Example C: "The Aztecs would gladly use shells and jewels to create dyes, but they would never use turmeric."
Example D: "In the fourteenth century, Aztecs would use shells and jewels to create dyes, toiling for untold hours to achieve what is now accomplished by simply boiling powdered cochineal insects. How fortunate we are to live in such an enlightened age."
Again, in the Example D above, you could substitute "used" for "would use" without greatly affecting the meaning. However, "would use" is favored by the author to call attention to the abnormality.
Edit for more examples:
Remember that "would" in this usage describes habitual, or typical behavior.
I'm going to drop the "only" from your final example, because the modifier makes the two statements roughly equivalent. There is more contrast without it.
He spoke of it when he was under a lot of pressure
This statement means he spoke of it at least one time, and he was under pressure at the time that he spoke of it.
It can be considered a discrete event, solidly in the past.
He would speak of it when he was under a lot of pressure
This statement means that whenever he was under pressure, he spoke of it. It suggests that this occurred more than once. The central meaning conveyed is not the past event(s), but the past behavior: activity that was typical, or which could be generalized. This is why we call it "habitual."
The behavior is typical, such that if you traveled back to the timeframe being discussed, the occurrence "would" happen again (conditional/future tense) if the correct conditions were supplied.
Here are a few more examples that may shed some light on the different scenarios:
Simple Past for Discrete event(s):
Last month, I stopped at a gas station for a bite to eat after work.
Seven times last month, I stopped at a gas station for a bite to eat after work.
Simple Past with a frequency for Habitual Behavior:
Last month I frequently stopped at a gas station for a bite to eat after work.
Would for Habitual Behavior:
Before getting married I would stop at a gas station for a bite to eat after work.
Would for Contrasting Habitual Behavior:
In previous years I would stop at a gas station for a bite to eat. Now I wait until I get home.
Would for Habitual Past Tense Conditional:
I would stop at a gas station for a bite to eat if it wasn't too cold outside.
And for the sake of completeness...
Would for Present Tense Conditional:
I would stop at a gas station for a bite to eat if it weren't so cold outside.
Would for Discrete Past Tense Conditional:
I would have stopped at a gas station for a bite to eat if it hadn't been so cold outside.
I had a car
In some situation or event in the past, you had a car. You may or may not have a car now.
I used to have a car
This emphasizes the fact you don't have a car now.
I was having a car
For have, this doesn't work in the sense of "possess". The reader/listener will assume you mean "consume" (in the sense of I was having dinner) and will laugh at you unless context is allowing you to be something that eats a car, which is not likely.
For other verbs - this is past progressive tense, and makes the action apply over a duration of time in the past - the typical reason for this construct is that you want to relate another event that happens in the middle of this duration, or interrupts the duration.
I was thinking of going to the park but I went to the store instead. (You spent some time deciding what to do.)
I was living in Chicago while the flood happened.
I was wanting some candy when she was here. (She was here for some duration and during that time, you wanted candy.)
Simple past simply says something happened and doesn't imply that it happened over any stretch of time.
Are past simple and 'used to' interchangeable?
Used to X means you don't have or do X now, and typically (but not always) implies that you had or done X a long time ago.
Best Answer
Hmm. I definitely prefer these:
This is talking in context of another action already implied to be habitual, such as "When he was on the hunt, Jack would..." 'Would' is used when you place an actor in a scenario and talk about the possibilities of that scenario, what the actor's wills and choices are. (Note: will -> would, using will in the choice or volition sense.)
This is tricky to explain. The problem is that English has tenses that merged together in the modern language, but that are still separate in their logic. I am not so sure 'simple past' is an accurate characterization of this verb. In an earlier tongue, this could be written as "the Aztecs of old did fight their enemies with wooden pikes." but modern sources consider that an intensifier instead of a tense creator. I'm stumped.
I am okay with 'would fight' in this context, but prefer it as I wrote it above.
"Used to" seems appropriate for two reasons: this seems like a storytelling thing instead of a more formal context, and he is also implied not to practice piano anymore. Used to should not be used in a formal context.
I prefer this simply because the sentence reads best when this isn't treated as a habitual at all. This is entirely equivalent to both "at some point, I read..." and "several times during my childhood, I read..." but the distinction isn't important, just that the event did happen.