The excerpt changes tenses because it's talking about things at different times. The first paragraph describes completed actions which took place in the past, hence the use of past tense. The second uses the present because it's talking about things at the current time or the recent past. The third begins by talking about the past, then proceeds to discuss the current state.
Think of the first paragraph like a big quotation. It's not describing any specific actual events - indeed, the second paragraph says that the first could describe several places - just some generic history. Because it's talking about history, it's in the past tense.
It sounds in the second paragraph is talking about the first paragraph. This is in the present because you're currently reading the article. Its words sound a certain way at the time you read them. Incidentally, I find this:
But this is also, and originally, Algeria, a quarter of a century earlier—the first major political crisis in the age of modern Islamism.
somewhat confusing; specifically, the aside about Algeria. I get the meaning (I think!), but I think the remark should be expanded a bit, and probably in the past tense.
The third paragraph opens by discussing completed actions in the past:
A flurry of freedom in the late 1980s gave way...
This first sentence sets the stage with some historical context. After giving us a bit information about the past, it goes on to describe the present state of affairs, appropriately shifting to the present tense to do so:
Today the country’s citizens remain powerless spectators...
Past, present and future match up with the times being described. Very generally speaking, you should work towards maintaining a single tense in your writing, especially if you aren't comfortable with the fine details of changing them. But there are plenty of reasons to change tenses. For example:
- Simple descriptions of events, as commonly seen in news, should be in the relevant tense (past events in past tense, etc).
- Literary foreshadowing might call for future tense: he gazed over his shoulder at her. He would never do so again.
- Dialogue written as spoken or thought by persons involved should be tensed as it normally would when speaking; e.g. she said, "I will go to the store tomorrow."
English verbs can't be inflected as strongly as in many languages. But as in many other areas, English makes up for this in many ways. The imperfect tense can be expressed in English; it just can't be expressed simply, by conjugating a verb.
As I understand it (as a rudimentary Spanish speaker), in Spanish, the imperfect tense indicates that the action of a verb took place, and was completed, before the present. So:
Yo caminaba
means that, at some unspecified point in time, I walked, repeatedly or for a long time, but that I don't necessarily do that type of walking any more.
Sometimes, in English, we express this meaning using the simple past tense, and the meaning is made clear by context. For example:
You were trapped in the library for eight weeks? How did you pass the time?
I read.
In Spanish, I'm guessing this would be expressed in the past imperfect. In English, the imperfective aspect of the verb--that it happened repeatedly or over a period of time--is not expressed in the verb, but it is clear from the context.
Sometimes, the imperfective nature of the action is expressed by the use of a phrase associated with the noun. So:
I ate raisins constantly.
is understood as an action taken repeatedly and completed in the past, because of the use of the adverb of time "constantly." Most of the examples of the Spanish imperfect I found also used a similar term--"a menudo," "cada dia". In English, combining one of these words with the past tense expresses the imperfect.
It's surprising how much can be expressed by context. I'm currently studying Chinese, which doesn't inflect its verbs at all and has no conventional tenses or moods at all. "You shi" can can mean "I am, "I was," or "I will be." When you ask a Chinese speaker how he or she can tell whether it's a statement about the past or future, the Chinese speaker will tell you: if I say "yesterday" it's the past, and if I say "tomorrow," it's the future. The same is, to a lesser extent, true for English. In most cases. even without the verb inflection, you can tell from context whether we're talking about the imperfect or the simple past.
Best Answer
I usually don't like to split my answer into two under the same question, but I believe this is a good exception.
To give you some quotes from a movie I like. Here are some parts of Prometheus (2012), which I especially like (no matter what others said about it), enough that I can see these scenes immediately and vividly just by reading these quoes. Also, they can help pointing out why Conditional I, II, and III, while being useful, are insufficient.
This is a good example of Condition Type I (or, according to PEU1 257.2, present tense with future meaning).
This structure of "if you're ..., you have reached ..." is a good example of something not Conditional I, II, or III. When I heard it, it was almost as if he should said "..., you would have reached ...", but he didn't. He simply said "..., you have reached ...".
This is a good example of Condition Type II (or, according to PEU 258.2, if + past; would + infinitive without to).
Another good example of something not Conditional I, II, or III. According to PEU 247.1, this "if you wanna ..., you (really) don't have to ...," is categorized as "the same tenses as with other conjunctions".
Another good example of something not Conditional I, II, or III. I am not even sure which item in PEU it would fall into. Perhaps 262.4, mixed tense: "Sometimes a simple past tense is used with if where a past perfect would be normal. This is more common is American English."
1. PEU = Michael Swan's, Practical English Usage.