A topic sentence tells the readers what the topic is and what the writer will say about the topic. Usually the topic sentence in the first or second sentence in the paragraph.
The topic is what a paragraph is about. The main idea is the point the writer is making about the topic. If we look at the topic sentence of a paragraph then, we should have a basic understanding of the main idea.
Apples are a favorite snack for many students.
APPLES is the topic. A FAVORITE SNACK FOR MANY STUDENTS tells us what the writer will say about the topic, and gives us a basic understanding of the main idea. What the writer says about the topic is also called the controlling idea.
[Topic + Controlling Idea = Topic Sentence] = Main Idea