After you've read the passage given to you, summarize what you've just read in your head or on a scrap of paper. What would you tell someone the paragraph was about if asked? You'd probably come up with a one-sentence explanation.
Chances are good that you've found the main idea if you can summarize the whole passage down to one sentence. Just be sure your one sentence is broad enough to cover every detail in the text.
At 8:00 Sally dropped her books in the mud on the way to school. At 11:00 she spilled milk on her clothes. At 4:00 she realized she left her homework at school. On her way to her bed, she knocked a lamp off her bedside table.
A new hearing device uses a magnet to hold the detachable sound-processing portion in place. Like other aids, it converts sound into vibrations. But it is unique in that it can transmit the vibrations directly to the magnet and then to the inner ear. This produces a clearer sound. The new device will not help all hearing-impaired people - only those with a hearing loss caused by infection or some other problem in the middle ear. It will probably help no more than 20 percent of all people with hearing problems. Those people who have persistent ear infections, however, should find relief and restored hearing with the new device.
Sometimes, the author of a paragraph (often new writers) will write the main idea directly in the text usually in the first few sentences. In that case, the main idea is easy to find: just look for the sentence that summarizes the whole paragraph.
Juan loves to play games. His favorite game is chess because it requires a great deal of thought. Juan also likes to play less demanding board games that are based mostly on luck. He prefers Monopoly because it requires luck and skill. If he’s alone, Juan likes to play action video games as long as they aren’t too violent.
The main idea here is "Juan loves to play games."
This is a little trickier. If the author doesn't write down the main idea of the text, it's up to you to infer what the main idea is. Basically, all you do is compose a sentence that is broad enough to cover every detail from the paragraph.
When you're with your friends, it's okay to be loud and use slang. They'll expect it and they aren't grading you on your grammar. When you're standing in a boardroom or sitting for an interview, you should use your best English possible, and keep your voice respectfully quiet. Try to gauge the personality of the interviewer and the setting of the workplace before cracking jokes or speaking out of turn. If you're ever in a position to speak publicly, always ask about your audience, and modify your language, tone, pitch and topic based on what you think the audience's preferences would be. You'd never give a lecture about atoms to third-graders!
Here, there is no main idea written, so you have to ask yourself, "What is the author trying to tell me?"
It seems to me that the author is giving us different situations (having an interview, hanging out with friends, speaking publicly) and then telling us to speak differently in each setting (use slang with friends, be respectful and quiet in an interview, etc.).
So, a sentence like, "People should speak differently in different situations" would fit perfectly as the main idea of that paragraph. We had to infer that because that sentence doesn't appear anywhere in the paragraph. But it was easy enough to do when you looked at the ideas as a whole.