Many grammatical or diction errors are easily forgiven, especially in English as it is spoken in North America. Because the countries of North America, such as Canada and United States—and less so Mexico—are largely immigrant cultures, there is a lot of tolerance for syntax errors, diction problems, and accent when speaking English.
Most speakers, and more importantly those trying to listen can ignore or work around utterances that in other languages would be incomprehensible. For instance, in the tonal languages words take on an entirely different meaning while the equivalent in English, the use of stress, can operate very differently and the word is still understood. In Thai, the word maa can mean mother, the verb to come, dog, and horse, depending on its tone. In English the word emptiness means the same whether it is said EMtiness, emTIness, or emtiNESS. It may be more difficult to parse for the listener, but they will be able to detect the outlines of the word spoken.
I was in an Algarve bus station in Portugal once trying to buy a ticket. I wanted to go to Olhão, which you can guess by the tilde is pronounced with a nasal sound. I tried a dozen different combinations before I resigned myself to writing it on a piece of paper. Once the ticket agent took the paper he made a point of pronouncing the name correctly, which was very little different from what I had said. The circumstance as particularly annoying because the other possible destinations—Albufeira, Portimão, Lagos, Tavira—did not sound even remotely the same. Part of the problem can perhaps be blamed on an irritable ticket agent, but at least part of it lies with a language which does not tolerate mispronunciation.
Other errors in English, such as commas, all over, the place, can be easily ignored, while the easy confusion between a semi-colon and colon is not universally understood in the language anyway so most people would not even notice problems in their use. The type of errors that stand out for English speakers are those associated with homophones. For some reason, the writer who confuses your with you’re and their with they’re make their readers suddenly and vociferously derisive. Although not every one of those gloating trolls online who find the mix-up between its and it’s would know the reason the word and contraction are an exception to the rule, they certainly can tell when it is used improperly.
The ease of detection has more to do with this derision than the issue with the error itself. The words are pronounced the same, therefore the problem is not one of misunderstanding. Instead, it is because the error is so easy for a native speaker to notice, they immediately are flooded with two very different feelings. Because they have so easily detected the simple error they instantly feel a satisfaction which can only come to people who are rarely given chances to flaunt their knowledge, which gives you a sense of their commitment to the subtleties of language, and they immediately feel an undeserved feeling of their own superiority. As we found from the Dunning-Kruger effect, the barely competent person has a greatly enhanced opinion of their competence. That they noticed such an obvious error is for them—online at least—proof of their skills with the language.
This means that if English is your second, third or even tenth language, you can afford to make some mistakes and still be understood, although you should avoid mixing up homophones. If you mistake were for we’re, which for witch, here for hear, are for our, buy for by, to for too for two, and hoard for horde, then you will be branded as stupid by those who are much more stupid that you are. You needn’t worry about garnering the respect of people who make judgements on the basis of such simple errors, but in case they are in positions of power over you—as your professors, prospective bosses, president, and landlords—it is worth realizing that even if their focus on the errors says more about them than you, you will have a difficult time shaking their judgement and it might have a devastatingly out-of-proportion effect on your life.