This article assumes a basic command of German.

There is a problem when translating an English text into German: that of determining exactly when the author is paraphrasing someone else or distancing himself from an idea or concept. English and German use different moods for that.

What are moods? They express the function of the verb or sentence and the writer's or speaker's attitude toward what he says. In English, regular old statements are formulated using indicative mood. For example:

Tom plays the guitar.

This sentence is in the indicative mood because it's presented as an objective fact.

Then there is the imperative mood:

Play the guitar, Tom!

Imperative moods are used to convey a command. Note that verbs are conjugated differently in this mood---here, it's "play," not "plays." Also, the imperative mood can only address others, meaning it exists only in the second-person singular and plural.

There is also the subjunctive mood:

It is imperative that Tom play the guitar.

This expresses the speaker's perceived necessity of Tom's playing the guitar. Note again the conjugation of the verb "play."

Another example of the subjunctive mood is:

If I were rich, I would buy a house.

In this case, the subjunctive mood expresses that the described scenario is not real. The speaker is not rich, but if he were, he would buy a house.

From what I know, these are the three main English moods. (There are other moods, but those need not concern us here.)

German likewise has the indicative mood:

Tom spielt Gitarre.

The infinitive of the verb is "spielen," and it's conjugated here in third-person singular, "spielt."

The imperative mood also works in German:

Spiel Gitarre, Tom!

Here, the verb is conjugated in the imperative mood, second-person singular.

Since both the indicative and the imperative mood exist in German, translating works well in both directions.

With the subjunctive mood, things are a bit harder. I'm tempted to say that there is no subjunctive mood in German, but that wouldn't be quite true. There appears to be a remnant of it of the form:

Lang lebe der König.

This example is not in the indicative mood because it doesn't say "lang lebt der König." That would be an attempt at a statement of fact (and a prophetic one at that because it makes an unknowable claim about the future). Instead, the speaker expresses the hope that the king will live long.

The same example exists in English:

Long live the king.

The same logic applies: a (prophetic) statement of fact would instead say "long lives the king."

This is all well and good, but it's the only example of the subjunctive I am aware of that works in German. If that is true, how does one write "if I were rich..." in German? The example with the house from above would read:

Wenn ich reich wäre, würde ich ein Haus kaufen.

Isn't this the subjunctive mood in German? "wäre" and "were" seem very similar, and they feel almost the same. But in German, this is called the Konjunktiv mood. There are two of them, and this one is Konjunktiv 2.

Konjunktiv 2 is likewise used to describe a scenario that isn't real. One can also use it to be polite:

Wärest du so nett, ...

which means

Would you be so kind as to...

or

Könntest du bitte...

which means

Could you please...

Especially in its use for counterfactuals the Konjunktiv 2 is so similar to the English subjunctive that one may call it the "past subjunctive" in German. However, I hesitate to call it that because most Germans don't know what a subjunctive mood is until they learn Spanish or French (where it has another function altogether, at least in Spanish). The topic may come up in English class, but I don't remember the subjunctive being mentioned explicitly when I went to school. Another reason is that Konjunktiv 1, from what I know, has no counterpart in English, not even in the subjunctive, and so it would be misleading to call it a "present subjunctive"---and yet I have seen it called that.

Which brings me to the purpose of Konjunktiv 1. It is used to convey indirect speech and/or distance oneself from what one is saying. For example:

Tom says his friend is nice.

In proper spoken and written German, this would not read:

Tom sagt, sein Freund ist nett.

but instead

Tom sagt, sein Freund sei nett.

"sei" is Konjunktiv 1 mood third-person singular of the verb "sein" ("to be"). It says "sei" and not "ist" because that's Tom's statement, not the speaker's. Konjunktiv 1 makes this explicit.

The conjugation of "leben" in the example "lang lebe der König" is equivalent to the Konjunktiv 1 present tense third-person singular of "leben." But I'd bet money that most native speakers of German would not think of that example when tasked with coming up with a sentence using Konjunktiv 1, and indeed would remark that the example feels different from all other uses of Konjunktiv 1 and that they would consider this a separate category altogether---which is why they may agree that it could be a remnant of an actual "Subjunktiv" that used to exist in German (?).

Konjunktiv 1 is especially useful when the speaker wants to distance himself from what is said. Imagine how useful this is in journalistic and legal contexts. Consider the following eyewitness account of a robbery in progress:

Augenzeugen berichten, der Täter beraube die Bank.

In English:

Eyewitnesses are reporting that the perpetrator is robbing the bank.

(For clarity, the English present progressive is merely turned into the simple present in German, but that's not important here.) The English sentence does not undergo a mood change, whereas the German one does. Any professional journalist would make that change. And while I am not an experienced translator, I believe it is important to recognize and make the change in translations, too.

Since English does not have a Konjunktiv 1 mood, accurate German-to-English translations will necessarily be lossy in those situations. It also means that going the other way round translators will need to carefully analyze what's explicitly said and what's implied to put the Konjunktiv 1 where needed.

Imagine you are tasked with translating the following, made-up sentences:

The idea that the natural sciences are only predictive has caused much damage. Because if the scientist believes that he must only predict, he loses the most important element: explanations.

All verbs are in the indicative mood. We can tell that the author is partially relaying information he disagrees with---he is not advocating for purely predictive but explanatory science---but it is on us to tell from the sentence structure and from certain cues with which information he disagrees and with which he agrees. I have highlighted those part-sentences that the author wants to distance himself from. Note how granular and interposed the shift can be.

The German translation makes the distinction explicit:

Die Vorstellung, die Naturwissenschaften seien ausschließlich vorhersagend, hat zu viel Schaden geführt. Denn wenn der Wissenschaftler glaubt, er dürfe nur vorhersagen, dann geht ihm oft das Wichtigste verloren: Erklärungen.

The highlighted part-sentences correspond exactly to the ones in the English original, but they use Konjunktiv 1.

There is a loophole which allows the translator to use Indikativ after all: the word "dass" ("that"):

Die Vorstellung, dass die Naturwissenschaften ausschließlich vorhersagend sind, hat zu viel Schaden geführt. Denn wenn der Wissenschaftler glaubt, dass er nur vorhersagen darf, dann geht ihm oft das Wichtigste verloren: Erklärungen.

Using "dass" works well and is grammatically valid. But without Konjunktiv 1, we're missing a key feature: grammatically explicit information about the author's intention. Also, using "dass" every time gets repetitive quickly; it's best to mix both styles. Luckily, it can be combined with the Konjunktiv 1: "dass die Naturwissenschaften ... seien."

In the above example, it is still relatively easy to tell that Konjunktiv 1 should be used because it contains cues: "Die Vorstellung" and "glaubt." Such cues need not be given, however, and frequently they are not. Consider this example:

According to this theory, science is predictive. Therefore, the answer is false.

The second sentence is tricky. Whose opinion is that? I see the two possibilities: either the author is presenting a conclusion which advocates of the theory he mentioned draw, or he is drawing a conclusion himself. If the former, it needs to be Konjunktiv 1; if the latter, Indikativ.

So how can the translator know which mood to use? He needs to understand the author's stance on the issue. When unclear, he can ask the author. In cases where the author is paraphrasing someone else and interspersing his own thoughts without always giving cues, which can span several paragraphs, the translator may need to check the source. In the above example, the translator needs to understand the theory's implications.

In other words, the translator needs to explicitly place Konjunktiv 1 when German requires it based on cues, context, and sources.