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How so? Have I been rude to you? Aren't you glad that somebody pointed to an obvious mistake in your written English, so that your writing can be better now? I am appaled at your reaction. Are you insecure or something?

English is my third language, by the way.

If you need me to explain perhaps you are not as educated as you think. It seems this is quite obvious: "Do they teach basic English grammar in your college?" You are being a sarcastic prick or just daft with that line of questioning OR just perhaps your command of correct English etiquette is seriously lapse.