Subordinate time clauses in Spanish are those clauses that are expressing a time when something happens, happened or will happen. These clauses will depend on the main clause, so they don't make sense on their own.
Time clauses refer to different points in time, sometimes they express simultaneous actions, previous actions or posterior actions, in relation to the action in the main clause.
Let's see some examples with some common time connectives:
"Siempre voy a Italia cuando llega el verano."
(I always go to Italy when summer arrives.)
"Recoge la ropa del jardín antes de que empiece a llover."
(Collect the clothes from the garden before it starts raining.)
"Te llamaré tan pronto como aterricemos."
(I will call you as soon as we land.)
"Siempre que estamos juntos nos peleamos."
(Whenever we are together we have an argument.)
"Tú podrías preparar los bocadillos mientras yo preparo las bebidas."
(You could prepare the sandwiches while I prepare the drinks.)
Not all time clauses follow the same structure. Some of them need the indicative, some the subjunctive, some the infinitive, depending on the nuance of each of them.