Just got curious, why is it that native english speakers tend to make mistakes with words like who/whom, they're/their, we're/were, to/too and so on, even more than those who speak it as a foreign language? I mean I can't think of any word in the finnish language which the finnish speakers would commonly mispell, so just find it a bit weird

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Because English slang makes people write sentences the way they say them. For example:
They're going to the circus (or) They are going to the circus.
= They going to the circus - if you're in a rush

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Also, one big issue, is SPELLCHECKER! Spellchecker and Grammar checkers are usually quite good, but when it comes to words that are similar and sound the same - they don't always pick them up.
Words that are words on their own, individually, and are spelt similar often go missed. For example:
- Who & Whom
- Were & We're & Where
- They're & There & Their
- It & Its & It's