Why isn't there preterite (pretérito indefinido) in the subjunctive mood?
Also, what do I use instead?
there is no preterite form for subjunctive, why? well generally because the time frame of the verb in the subjunctive is not the same as in the indictative, for example "fuera" does not necessarily refer to the past.
But take for example : "I'm happy that you came yesterday."
It could use a preterite subjunctive.
That is an imperfect meaning, compare: veniste ayer vs. estoy feliz que vinieras ayer. In the second sentence that act of coming is only relavent in the sense of making me happy, whereas in the first the act of you coming is what matters hence it uses a preterit form.
modified the answer a bit, so hope it makes sense now, but the imperfect does not refer to direct discrete completed actions, which the preterit does.
In relation to your comment below, all i can say is 2 things, 1. by adding I'm happy that meansing the following clause is now a subordinate and basically acts as a description for the main clause. 2. if you want to look at it that way, great, but just remember there is no´preterit subjunctive form in spanish, so i guess good luck
Fine...take any preterite sentence you can think of and add "I'm happy that" before it.
For example : "I'm happy that you ate dinner with me this evening."
It's complete and it happened one time in a specific time frame.
In this case, Spanish could use preterite subjunctive.
Depending on the context, you would use present perfect subjunctive, imperfect subjunctive, or past perfect subjunctive. I don't know why Duolingo doesn't teach the present perfect subjunctive or past perfect subjunctive.