A grammatical term that scares some people off is "subordinate clauses", probably more because of the name than what it means: it sounds more complicated than it is.
When the Key Stage 3 Framework asks us to "explore" subordinate clauses, it's not as mysterious as it might sound.
One of the most common subordinate clauses is the because-clause:
* "He ran because he was late."
The main clause gives you the main information: he ran; the subordinate clause answers the question: why did he run? Familiar stuff. But a text which used because-clauses all the time would quickly bore us because it would be so repetitive.
Of course our students need to be able to express causal relations - to show that one event happened because of another - but we need to equip them with some alternatives to the simple "event-because-event" pattern.
Rather than dish up ready-made examples, get your students to think of some ideas. You'll quickly find that they come up with other ways of signalling the causal link between being late and running.
A really easy one is "so": "He was late, so he ran." This KS2 pattern treats the cause and the effect as grammatical equals, so you could even insert "and" before "so": "He was late, and so he ran." Not a great pattern, but a useful alternative to "because".
One of the growth points at KS3 is when students start using two other subordinate-clause markers: "since" and "as". These are a bit tricky, because they both have another possible meaning, to do with time. For example, suppose they write: "I've bought the magazine since the new series started". Do they mean that they buy it because of the series, or simply that their new buying habit happened to coincide with its start?
Similar questions arise with "as". Think about: "We came in as it started to rain." This ambiguity is usually harmless because the meaning is obvious, but it can cause problems, so writers should be warned.
There are other ways of showing cause-effect relations, but they all raise the danger of ambiguity. One is the KS1 approach: simply put the cause in one sentence and the effect in the next one: "He was late. He ran as fast as he could, but..." Very respectable and effective - when used well.
Another one is more likely to be found at KS4: "Being late, he ran..." The participle "being" signals subordination instead of "because" - it's pithily short and very sophisticated.