Final answer:
The sentence lacks a comma after the introductory clause, making the punctuation incorrect; it should be "When the thunderstorm began, we ran for shelter".
Step-by-step explanation:
The punctuation in the sentence "When the thunderstorm began we ran for shelter" is incorrect. It lacks a necessary comma after the introductory clause. The proper punctuation should be "When the thunderstorm began, we ran for shelter". Recognizing correct punctuation involves determining where commas, semicolons, and dashes are correctly or incorrectly used.
When assessing the correct use of punctuation, the general rules include using commas after introductory elements, separating independent clauses with a comma before a coordinating conjunction, and avoiding comma splices and sentence fragments.