Choose a Sentence Boundary by Testing Both Clauses

Try this original punctuation question. The sample looked clear, blank, however, the sensor detected impurities. Before choosing a mark, test the words on each side of the blank.

The sample looked clear can stand as a sentence. So can the sensor detected impurities. Both are independent clauses. However expresses contrast, but it does not join clauses in the way that and or but does.

A semicolon separates the two complete clauses. The comma already after however sets off that transition. A comma alone would create a comma splice. No punctuation would fuse the clauses. A colon would promise an explanation of clarity, but the second clause instead contrasts with it.

Now change the beginning: Although the sample looked clear, the sensor detected impurities. Although makes the first clause dependent, so an introductory comma works. Do not apply the semicolon mechanically. Test the clauses again whenever the wording changes.

Read the sentence first: The sample looked clear ___ however, the sensor detected impurities. Choose: comma, semicolon, no punctuation, or colon.

Find two complete clauses: The sample looked clear. The sensor detected impurities. Each has its own subject and finite verb.

Build the boundary: The sample looked clear; however, the sensor detected impurities. A comma alone would splice two complete clauses.

Transfer: change the structure: Although the sample looked clear, the sensor detected impurities. Although makes the first clause dependent. Now a comma works.

Complete the boundary: Because the first measurement was inconsistent ___ the researcher repeated the trial. Choose a comma or semicolon and explain.

Answer: A comma. Because makes the opening clause dependent. A semicolon would not join that dependent clause to the independent main clause.