If you know a word, you can't unhear it

Horse.

Big animal with a mane and long tail, big distinctive head large body with long thin legs. That word can't be anything else.

For a native speaker, any word in your language is like an automatic switch. It says what it is, there is no other thing it could be but what it says. It stuffs itself into your mind and there's no unstuffing it.

When learning a language, a word has this ethereal feel. It could mean something else, it could mean, well I'm not sure, is it some sort of ... no, is it ... bread? I'm sure it was an animal...or  maybe not.

This goes for syntax and phonology too. Lots of leniency as a learner, there is no wiggle room at all for a native speaker. For a native speaker the slightest deviance sticks out as something entirely different. Suppose you mean 'I bet a dollar', but you said 'I bit a dollar'. Why would you bite a dollar, that is crazy! Even if you're at a gambling table, people would get all bent out of shape. "Did you mean 'bet' or 'bit'. No one bites dollars here they only bet. Did you really bite a dollar?".