What is the subordinate clause in the following sentence?: He was late to school because of the snowstorm.
The subordinate clause in this sentence is " because of the snowstorm ".
Here's a definition of clause (slightly redacted) from
Definition of clause
A group of words containing a subject and predicate and functioning as a member of a complex or compound sentence.
The sentence "When it rained they went inside" consists of two clauses: "When it rained" and "they went inside."
[End of quote]
"When it rained" cannot stand alone: When it rained . . . what? What happened when it rained?
Again, from the Merriam-Webster site:
Definition of subordinate clause
A clause that does not form a simple sentence by itself and that is connected to the main clause of a sentence.
In the sentence "I went home because I felt ill," "because I felt ill" is a subordinate clause.
A subordinate clause has a subject and verb, but cannot stand alone as a sentence, because it begins with a Subordinate Conjunction or a Relative Pronoun, that connects (subordinates) the clause to the main one.
Check out this site for a really good list of subordinating conjunctions, relative pronouns and examples (and it's printable):
N.B.: There is no verb in " because of the snowstorm", so it can be considered a "verbless clause".
[See: https://www.thoughtco.com/verbless-clause-1692588 ]
If your sentence above were " He was late to school because the snowstorm was heavy", then the subordinate clause would be more obvious.