Konjunktiv I:

You use the subjunctive I to express probabilities and within fixed expressions.

Konjunktiv II:

The second form of the subjunctive is used more often.
You will find this verbal form when ...
... you express your hopes or wishes
... you negotiate unrealistic terms
... you politely ask for something
... you want to give advice
... you make a comparison, using the term „als ob“

In the present tense, you must distinguish between regular and irregular verbs if you want to form the subjunctive correctly:

Would you like to use the subjunctive II in the past?

Then use the form of "have" or "be" in subjunctive II together with the past participle of the verb.
Example:
ich hätte gelacht
ich wäre gegangen