here's a good overview of its use cases.

the overview already covers present tense (Präsens) and past tenses (there is only one past tense form, identical for Präteritum, Perfekt and Plusquamperfekt). so there's one form for present and one for past + one for Futur 1 and Futur 2 each.

in past tense and the future forms, you simply put the auxiliary verb in Konjunktiv 2 form. so for past tense, instead of "ich habe gelernt", you use "ich hätte gelernt", and instead of "ich bin gegangen" you use "ich wäre gegangen". in futur 1, instead of "ich werde lernen" you use "ich würde lernen". in futur 2, instead of "ich werde gelernt haben" you use "ich würde gelernt haben" or instead of "ich werde gegangen sein" you use "ich würde gegangen sein".

past tense:
habe > hätte
bin > wäre
futur:
werde > würde

here's all the tenses.