l As you can see in the example Juan quiere que su
hermana cante con él. (Juan wants his sister to sing
with him), there must be two people involved in
this situation, Juan and his sister, one in the main
clause and one in the subordinate clause.
Therefore, if there isn't a second person involved,
there is no subjunctive but an infinive and there is
no "que".
l Juan quiere que su hermana cante con él. (2
subjects = subjunctive and "que") but:  Juan
quiere cantar solo. (one subject, Juan = no
subjunctive but an infinitive and no "que")