The part "When they arrived at beach", of the sentence indicates, that it is an adverbial dependent clause.
Adverbial Dependent Clause: A Dependent clause is that changes the main verb of an independent clause. Adverbial phrases always begin with a subordinating conjunction and must be associated with an independent phrase to make sense.
Conjunction:
A part of speech that joins sentences, words, or phrases called a conjuncts of conjunctions. Examples of conjunctions are and, but, or, yet, because, unless, or, and still. There are three types of conjunctions: correlating, coordinating and subordinating.
Clause:
A group of words containing a subject and a verb that have a relation or component involving the semantic predicate and the semantic predicate. This relationship is very important. Rather than just randomly grouping words, clauses convey information about what the subject is or is doing.
"when they arrived at the beach" is an adverbial dependent clause.
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