Don't know if you given it any thought.

But a beginner will struggle with most programming tasks.

When you get more experience, you will be able to solve many problem without really thinking about it. Similar to how you can talk without first having to sit down and consider what the next sentence should be.

When you get to that stage, you also learn to see patterns in code. You can look at source code and recognize from the coding style who have written the code. Just like people are reading books and discussing the writing style of different authors.

Teachers sees a lot of turn-in tasks. They can recognize good or bad solutions. They can recognize which student is likely to turn in a good or bad solution. They can ask questions that instantly shows if the student has written - and truly knows - the code that was handed in. They can recognize code sections that does not fit the original problem description, but are not added as a direct improvement but instead looks to be remains from a different program written to fulfill other requirements.

So yes - if you do copy/paste code and hand in as your own, there is a very significant chance that you'll get into troubles convincing the teacher that it is your code.

Besides that, lots of schools also makes use of automatic tools that compares code sections or sentences from hand-ins with code or text available on the net. Based on the percentage of similarity, the teacher will be notified and will then be able to make a more specific check of that students work.

Besides that, more than one teacher are reading programming forums. Both to catch cheating students but also because they can pick up own knowledge from following programming forums.

Edit: One thing I intended to write but forgot, about programming styles: When you do learn to read source code to a level where you notice all the tiny details of different programming styles, it will stick out as a sore throat when different parts of a program have been written by different people. It is normally visible even if the different people are programming using the same rules for indentation, braces, parentheses etc. There are too many tiny details that will be picked up by a good reader - often without the person actually thinking about them. Even if trying to normalize the code, lots of hints will survive and make code blocks feel as written by different people. Just think about all the tiny details that makes you recognize someone elses voice. Or what details is it that makes you recognize a friends face? Even when you are unable to write down what features that makes you recognize the voice or face, your brain will notice them. Same with source code - you recognize differences even when you can't put your finger on the minute differences your pattern-processing brain have evaluated.