A small lesson.
A clearer next step.

The main idea is the point a passage develops. The topic names the subject. Details explain or support what the writer says about that subject.

Here is an original short passage. A library added evening hours. Adults came after work. Its study rooms were busier in the evenings.

The topic is library hours. A main idea is that evening opening gave people more opportunities to use the library.

The adults and the busy study rooms are supporting details. Neither one alone covers the entire passage.

A choice saying every library must open all night goes too far. The passage does not make that claim or provide enough evidence for it.

Look at the first and last sentences, but read the middle too. A main idea may be implied rather than written in one sentence.

Ask whether your answer fits the whole passage. Reject answers that cover only one detail, or add a claim the writer never supports.