Tax law is complex and this is a place for oversimplification, so it's hard to answer in detail. Even the concept of "taxes" varies a lot by jurisdiction.

But in general, if a tax is an "income tax" how much you pay depends directly on how much "income" you have. I put it in quotes because the definition of "income" is going to be the concern of an awful lot of the relevant tax law. It's more complex than just "how much money you made".

For example, if you own a business, it's understood that you have to use some portion of your money on business expenses like salaries. Since those salaries become employee income that gets taxed, that's some money that you can subtract from your business's "income". There are a lot of other categories of expenses that the law decides is worthy of removal from your "income".

Usually a "write off" is some thing that costs money that you normally wouldn't buy, but if you spend money on it that money gets subtracted from your "income" and creates an advantageous situation.

One way that happens is when taxes are "bracketed". For example, a very simple "bracketed" income tax might say you pay 0% on up to $10, 1% on $10-$100, and 10% on $100-$1000. So if you get paid $120 for something, you pay:

• $0 on the first $10 ($140 remains)
• $1 (1%) for the next $100 ($10 remains)
• $1 for the remaining $10 (at the 10% tax rate)

So in this case, you might really want to try and find a way to "write off" $10 of your income. If you can buy something that lets you deduct it from your income, your $110 will only get taxed $1 instead of $2 for the $120. You "lost" $10 on some business expenses, but it saved you $1 in taxes so it's more like you got a 10% discount on that expense, in the big picture. In real-life scenarios, sometimes a write-off means a person goes from paying taxes to getting a refund!

That's the kind of situation where people consider write-offs: sometimes spending a little bit of money can dramatically reduce how much you owe in taxes.

It is different for students, corporations, and independent workers, but that's mainly because:

• Students don't generally have a lot of income, and likely have a lot of debts, so they don't tend to have complicated income scenarios or owe a lot of taxes unless they're very well off. The things they can write off tend to be related to loan payments/interest.
• Corporations have a lot of assets, a lot of expenses that can be write-offs, and the resources to hire lawyers and accountants that let them structure their "income" in different ways.
• Independent workers are basically like small corporations: common write-offs are money spent for office space (even if you build a home office, it can be deducted from income if it meets certain criteria), equipment, supplies, etc.

But you could also say it's "the same" for all of them, because the idea is we want to tax people who make a lot of money and spend it on luxury more than we want to tax people who make a lot of money but invest that money back into business ventures and we DON'T want to tax people who aren't making a lot of money. This gets kind of cloudy and frustrating because it means rich people get a lot of ways to not pay taxes on income, and the way companies "invest" the money doesn't always benefit their workers the way the system intends. People in the middle don't tend to have as much flexibility and it can be frustrating to hear that a man is worth $200 billion but gets to publicly brag that he's not sure if he plans on paying any taxes.