Whereas the subjunctive I forms of the verbs are based on the infinitive form (sei from sein, habe from haben, etc.), the subjunctive II forms are based on the simple-past forms of the verbs.
The reading challenge for you is that whereas subjunctive I verb forms are always clearly, visibly unique to subjunctive I, subjunctive II forms are only distinctively unique for some irregular verbs. For regular and some other irregular verbs, the subjunctive II present-tense forms are completely identical to the indicative (normal) simple-past verb forms. Fortunately, for this very reason, it is for irregular verbs that you will more often encounter subjunctive II usages.
Taking the irregular verbs haben and sein as examples, note that the verb endings are all the same as for subjunctive I (which is a helpful cue) and that the roots are the verbs’ simple-past forms (which is a similarity you’ll need to watch out for):
|1st||ich||hätte / wäre||wir||hätten / wären|
|2nd||du||hättest / wärest||ihr||hättet / wäret|
|3rd||er/sie/es||hätte / wäre||sie/Sie||hätten / wären|
For these two very common verbs you can also see that the easily recognizable differences from the simple-past verb forms (e.g., hatte / war) are: 1) the umlaut and 2) – in some cases – the signature additional syllable (spelled with –e-) that is shared by subjunctive I and II. For other irregular verbs, consult the irregular-verb chart in your dictionary to check whether a particular verb form you see is simple-past indicative or present-tense subjunctive II.
For regular verbs and any irregular verbs that have no such visible distinction, you will need to consider context, such as the Wenn …, dann … construct, to make that reading decision.
Wenn er viel Geld hätte, (so) reiste er nach Deutschland.
If he had a lot of money, he would travel to Germany.
Wenn sie reich wäre, (so) kaufte sie ein neues Haus.
If she were rich, she would buy a new house.
Wenn ihre Eltern kämen, (dann) gingen sie ins Restaurant.
If their parents came, they would go to a restaurant.
Here’s a time-saving tip to help you recognize subjunctive II for some common irregular verbs whose vowels do not take an umlaut to signal subjunctive II mood: Verbs whose past-tense, singular, 1st- and 3rd-person forms do not end with an –e, such as gehen (ging), bleiben (blieb), etc., do get an –e ending in their subjunctive II form. The top example here is in indicative past tense, and the lower one is in subjunctive II present tense:
Ging er ….? Ich blieb….
Did he go …? I stayed ….
Ginge er …. Wenn ich bliebe, ….
If he were going … If I were to stay …
As with subjunctive I, the subjunctive II past tense follows the model of German present-perfect tense. (So there are no “simple past” subjunctive forms at all.) So when you encounter a present-perfect construction which uses the above subjunctive-II forms of the auxiliary verb haben or sein instead of their normal present-tense indicative forms, then you are dealing with past tense subjunctive II mood.
Wenn sie nach Deutschland gereist wären, (so) hätten sie den Kölner Dom gesehen.
If they had travelled to Germany then they would have seen Cologne cathedral.
Wenn der Ingenieur die Maschine richtig repariert hätte, (dann) wäre der Unfall nicht geschehen.
If the engineer had repaired the machine correctly, (then) the accident would not have happened.
See how this same pattern works even when a modal verb is involved:
Ich hätte das gestern machen sollen.
I should have done that yesterday.
Wir brachten unsere Schlafsäcke mit, falls wir übernachten hätten müssen.
We brought our sleeping bags along, in case we would have had to stay overnight.
Can you construct how the above examples would appear if they were indicative (normal) statements instead of subjunctive II?