What is a dangling modifier?

A modifier that is not attached to the thing it is supposed to describe, so the sentence appears to say something the writer did not mean.

Walking to the station, the rain started.

Grammatically, walking to the station attaches to the nearest subject, which is the rain. The sentence says the rain was walking. The writer meant that they were.

The fix is to give the modifier something correct to attach to:

Walking to the station, I was caught in the rain. As I walked to the station, it started to rain.

The pattern to watch for is a sentence opening with a participle or a descriptive phrase, followed by a comma. Whatever comes immediately after that comma is what the phrase will attach to, whether the writer intended it or not.

More examples of the same error:

After reviewing the data, the conclusion was obvious. — the conclusion did not review anything. Having finished the report, the printer jammed. — the printer did not write it. At six years old, my family moved to Leeds. — the family was not six.

These are often funny, which is why they persist as examples, but the real cost is a moment of confusion for the reader.

A related error is the misplaced modifier, where the phrase has something to attach to but sits next to the wrong word. She almost drove her children to school every day means she nearly did it; she drove her children to school almost every day is presumably intended. Only is the word most often misplaced in English.

The reliable check: read the opening phrase, then ask who or what is doing it.