"Subordinate clause" is one of those terms that appears on school reports and homework sheets with zero explanation, as if every parent already knows it. Most don't — here's the plain version.
The simple definition
A subordinate clause is a part of a sentence that has a subject and a verb (like a full sentence does) but can't stand alone — it needs the rest of the sentence to make complete sense.
"Because it was raining" has a subject (it) and a verb (was raining), but reading it alone leaves you hanging — because what happened? It needs a main clause attached: "We stayed inside because it was raining."
Main clause versus subordinate clause
Every sentence with a subordinate clause also has a main clause — the part that genuinely could stand alone as a complete sentence. In "We stayed inside because it was raining," the main clause is "We stayed inside" (makes sense alone) and the subordinate clause is "because it was raining" (doesn't).
How to spot one together
A useful trick: cover up part of the sentence and check if what's left still makes sense on its own. If it does, you've covered the subordinate clause. If what's left is confusing or incomplete, you've covered the main clause instead.
Words that often signal a subordinate clause
because, although, since, when, if, while, unless, before, after — these subordinating conjunctions almost always introduce a subordinate clause, which is a genuinely reliable spotting trick for a child still getting used to the concept.
Why this matters for writing, not just tests
Understanding subordinate clauses helps children write longer, more varied sentences instead of a string of short simple ones — and it's directly assessed in the KS2 grammar test, so genuine understanding (not just spotting the right word) pays off twice over.
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