When the subject of the principal clause (the clause with the verb that expresses desire) and the subject of the subordinate clause are the same one, the verb in the subordinate clause it's an infinitive. When you have different subjects, you have to use the subjunctive. For example: -¿quieres irte a casa?..here we have "quieres"+subordinate with the same subject =infinitivo -¿quieres que él se vaya también?..quieres+different subject=subjunctive -quiero visitar Islandia cuando vivamos en España..same subject (here the subjunctive appears with "cuando" because it's a temporal clause expressing a future situation). -quiero que visitemos Islandia cuando vivamos en España..different subjects -quería sentarme..(quería=yo sentarme=yo) -quería que se sentara (quería=yo, que se sentara=él) you have to consider this construction as a two-part sentence. with the verb "querer" you have the the main clause. Than it appears another verb, so it's another clause, the subordinate one. With same subjects you use infinitive, with different ones you use the subjunctive introduced by the conjunction "que". I hope I helped you.

¿Te gustaría ir a casa pronto? vs ¿Te gustaría que él fuera también? The others examples you are giving: Quiero visitar Islandia cuando vivamos en España / Quiero que vayamos a Islandia cuando vivamos en España Quería sentarme - Quería que se sentara