German has two forms of the subjunctive: the general and the special subjunctive. The general subjunctive is the one native English speakers think of when referring to the subjunctive (e.g. could, would, should). This grammatical mood, referred to in German as the Konjunktiv II, indicates hypothetical or unreal/imaginary situations, including wishes and desires.
Ich könnte einen Marathon laufen, wenn ich fit wäre.
I could run a marathon if I were in shape.
The modal verb ‘könnte" (could) is the subjunctive form of ‘können" (can), and "wäre" (were) is the subjunctive form of "sein" (to be). In this case, it is not a question of whether the speaker is physically able to run a marathon in his/her present condition or level of fitness; rather, it is speculation based on a hypothetical level of physical fitness.
Typically, we find hypotheticals posited as conditionals (e.g. If X, then Y). Similarly, in German, Konjunktiv II is often expressed using the subordinating conjunction ‘wenn’ (if):
Wenn ich reich wäre, könnte ich mir einen Ferrari leisten.
If I were rich, I could afford a Ferrari.
In the present tense subjunctive, there are three forms to learn: ‘hätten,’ ‘wären,’ and ‘würden’+ Infinitiv.
‘Wären’ (were/would be) and ‘hätten’ (had/would have) can stand alone in the present tense subjunctive. The modal verbs and ‘würden,’ however, require an infinitive form of the main verb at the end of the sentence or clause. For example:
Ich würde Fussball spielen, wenn ich Zeit hätte.
I would play soccer I had (the) time.
The ‘würden’ + Infinitiv form is used for all regular verbs and often for most strong verbs, too, despite the latter having their own forms. The reason for this is that the subjunctive and imperfect forms of regular verbs are identical (e.g. spielte / spielte), which makes it difficult to differentiate between the two, so often "würden + infinitive" is used.
Below is a chart of the modal verbs in Konjunktiv II, which only differ from their Imperfect forms insofar as umlauts present in infinitive forms are added back to the stem vowels.
Te translation for modal verbs in the Konjunktiv II would be:
"wollten" -> would want, "könnten" -> could, would be able to, müssten -> would have to
"sollten" -> should, dürften -> "would be allowed to", and "möchten" -> would like to
Er müsste täglich trainieren, wenn er besser werden will.
He would have to practice every day, if he wants to get better.
Ich könnte dich morgen anrufen, wenn du Zeit hast.
I could call you tomorrow, if you have time.
In contrast, look at the exact same sentences, but in the present tense, and not in the Konjunktiv II:
Er muss täglich trainieren, wenn er besser werden will.
He must practice everyday, if he wants to get better.
Ich kann dich morgen anrufen, wenn du Zeit hast.
I can call you tomorrow, if you have time.