I’m going to work through your example sets in reverse order.
First the general usage of articles: A/an vs. the.
A and an normally refer to a noun, but not a specific/particular/definite item from all available count nouns of that type. These are known as indefinite articles.
https://www.quickanddirtytips.com/education/grammar/a-versus-an?page=1
“Grab me a Sprite, would you?”
There are 5 cans of Sprite in the fridge and it doesn’t matter which can you bring.
The normally refers to a particular/specific/definite noun. This is the definite article.
“Would you bring me the Sprite from the fridge, please?”
There are 5 cans of Sprite, but only 1 of them is chilled. The cold one is wanted.
How does this relate to your examples?
The tiger is without doubt the most magnificent of the big cats.
A tiger is a particular type of big cat.
I suspect what’s happening here is that wildlife shows (or whomever else is using this construction) tend to have implied words. When they say “The tiger is...” they really mean: “The tiger species is the most magnificent of the cat (Felidae) genus,” or something like that. But they don’t want to use any jargon words, so they simplify and it comes out slightly awkward.
Still the same rule, though.
However, there would be nothing wrong with saying:
“Tigers are the most magnificent of the big cats.”
And that would probably be better.
The computer has changed everyone’s lives in so many ways.
Personally, I think this would be better:
“Computers have changed everyone’s lives in so many ways.”
I guess in the example there might be implied words in there like:
“The compluter is an innovation which has changed everyone’s lives in so many ways.”
And that would make it a specific type of innovation.
But I think this is bad style. Why? Because you’ll sometimes run into problems. People don’t like to be called “the disabled” and so forth. And the modern trend is to cut out unnecessary words and make the style more straightforward, and those read like an old-fashioned style.
complicated dances like the tango
This is the best of all these examples. A tango is a very specific type of ballroom dance. It is used most often with the definite article:
“They are dancing the tango.”
“They are dancing the waltz.”
Because it’s not just that a couple or some person is dancing. The specific dance is identified.
So let’s look at what the rule guide said about this construction with examples featuring the tiger, the computer, and the tango.
10 used before a singular noun when you are referring to a particular type of thing or person in a general way
1) First requirement is that the noun must be singular.
2) Second requirement is the same for any usage of the: you must be referring to a particular/definite item or type of item.
3) Third is that you’re using this specific singular noun to make a generalized reference (usually to the entire class that specific item belongs to).
Ok, so let’s go back to that first example and check it against this list of requirements.
Life is a dream
1) Singular noun? Not really. In this construction, it’s not talking about any one instance of life, but life as an idea. This isn’t my life or their lives or your life but just life overall.
It’s like time. (Not time of day or the time of the meeting, but time from the time-space continuum.) Or like gravity.
A concept that doesn’t have multiple instances on this world.
2) No for the same reasons as in 1).
3) Not applicable because 1) and 2) not met. Already a generalization.
Ok, second set.
A woman without a man is like a fish without a bicycle. (old feminist joke)
A baby deer can stand as soon as it is born.
These fail on item 2). No specific woman, fish, bicyle, or baby deer (fawn!) is meant, so the definite article cannot be used.
If we substitute rule 2) with:
Second requirement is the same for any usage of a/n: you must be referring to any one of a type of item (not a particular one).
Then it would meet all 3 rules.
Does that make sense?
