It is the first sentence of a paragraph which is the topic sentence.
mainly the topic sentence in the paragraph
Topic sentence
The topic sentence should state the main idea that will be developed. The reader should easily be able to see the main idea, simply by reading the topic sentence. The topic sentence is usually the first sentence in the paragraph, and is the most important thought in the paragraph.
The topic sentence summarizes the main idea of a paragraph and supports the thesis.
If you mean what sentence in the paragraph contains the main idea then it would be the topic sentence and the rephrased version of the conclusion.
if a paragraph does not have a topic sentence then
simple sentence with the main idea.
Topic sentence
a topic sentence
The sentence that introduces the paragraph's main ideaa sentence that introduces the subject of the paragraph.
Implied Main Ideas sometimes,there is no topic sentence in a paragraph. this does not mean,that there is no main idea. (implied) (:-)
It restates the main idea of the paragraph using different words.