You should determine the main idea by combining the supporting details.

The top three tips are as follows:  Look at the big picture, Re-read the first and last sentence, and look for repetition.

The main idea can be implied or stated.

There are many fun things to do at the beach.  Swimming is one thing that can be done at the beach.  Snorkeling is another thing that can be enjoyed.  Playing beach volleyball can be a lot of fun.  It is also fun to look for shells.  Some people simply like to sunbathe.
What is the main idea?

Benjamin Franklin loved to write.  When he was twenty-two, he set up his own printing shop and started a newspaper.  He also printed a book called Poor Richard’s Almanac.  It had funny stories, information about the weather, and wise sayings.

I can't wait until this weekend! We’re going to my grandparents’ house and my entire family will be there. I will spend most of my time playing games with my cousins. We always have such a good time together! The day goes by so fast, and by the end we are all exhausted... but we still don’t want to leave!
What is the main idea?

Sharks can lose up to 30,000 teeth in their lifetime. Sharks have teeth arranged in rows inside their mouths. These teeth do not have roots like human teeth do, so they fall out easily. When a shark takes a powerful bite into its food, several teeth are lost. When these teeth are lost, new ones push forward from the back rows to replace them. Luckily, sharks continually grow new teeth to replace the thousands they lose.

Training a dog to be a watchdog often reduces unexpected results.  Some dogs quickly learn the difference between unwanted people and friends.  This is because their masters welcome friends into their houses.  However, some dogs will always attack the postman who comes to deliver letters.  One explanation for this behavior is that, although the postman comes to the house often, he never enters the house.  Therefore, the dog thinks the postman is someone not wanted, but keeps coming back anyway. Which sentence is the main idea?

Someday we will all have robots that will be our personal servants. They will look and behave much like real humans. We will be able to talk to these mechanical helpers and they will be able to respond in kind! Amazingly, the robots of the future will be able to learn from experience. They will be smart, strong, and untiring workers whose only goal will be to make our lives easier.
What is this selection mainly about?

There are many fun things to do at the beach.  Swimming is one thing that can be done at the beach.  Snorkeling is another thing that can be enjoyed.  Playing beach volleyball can be a lot of fun.  It is also fun to look for shells.  Some people simply like to sunbathe.
What is the main idea?

Sharks can lose up to 30,000 teeth in their lifetime. Sharks have teeth arranged in rows inside their mouths. These teeth do not have roots like human teeth do, so they fall out easily. When a shark takes a powerful bite into its food, several teeth are lost. When these teeth are lost, new ones push forward from the back rows to replace them. Luckily, sharks continually grow new teeth to replace the thousands they lose.

You should determine the main idea by combining the supporting details.

The top three tips are as follows:  Look at the big picture, Re-read the first and last sentence, and look for repetition.

The main idea can be implied or stated.

Benjamin Franklin loved to write.  When he was twenty-two, he set up his own printing shop and started a newspaper.  He also printed a book called Poor Richard’s Almanac.  It had funny stories, information about the weather, and wise sayings.

I can't wait until this weekend! We’re going to my grandparents’ house and my entire family will be there. I will spend most of my time playing games with my cousins. We always have such a good time together! The day goes by so fast, and by the end we are all exhausted... but we still don’t want to leave!
What is the main idea?

Training a dog to be a watchdog often reduces unexpected results.  Some dogs quickly learn the difference between unwanted people and friends.  This is because their masters welcome friends into their houses.  However, some dogs will always attack the postman who comes to deliver letters.  One explanation for this behavior is that, although the postman comes to the house often, he never enters the house.  Therefore, the dog thinks the postman is someone not wanted, but keeps coming back anyway. Which sentence is the main idea?

What is the main idea?

Someday we will all have robots that will be our personal servants. They will look and behave much like real humans. We will be able to talk to these mechanical helpers and they will be able to respond in kind! Amazingly, the robots of the future will be able to learn from experience. They will be smart, strong, and untiring workers whose only goal will be to make our lives easier.
What is this selection mainly about?