You could read the section on subjonctive in Wright's First Latin steps
. The explanations are nice and simple and followed by more than 700 short sentences; this should give you some theoretical background as well as plenty of practice.
The book Pons Tironum
could also be very useful to you. The first sections are devoted to the different use of the subjunctive and consist of short stories repeating over and over the same construction.
Pons Tironum, p. ix wrote:(...) "Nam semper tibi imperat ut tunicam apud lectulum tuum deponas." Ego tamen iratus ei ut taceat impero. Tacet igitur et alii imperat servo ut tunicam novam mihi det. Servus ille alteram tunicam adfert, et mihi "Davus" inquit, "ut hanc tunicam tibi darem mihi imperavit. Imperavit quoque ut celeriter te amicires." (...)
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