In many situations people will have a negative opinion about someone who makes subject-verb agreement errors. In academic and professional writing and presentations, those errors are not acceptable because they don't follow the rules for Standard English (and those rules might change from country to country, but within a country they are pretty stable). Outside of these kinds of formal situations, subject-verb agreement errors are not important. In pop music, "street English", and internet chat, etc. it's perfectly fine to use the non-standard forms. It all depends on YOU. Do you need to write a business letter? Most readers will notice your subject-agreement errors (and other errors as well) and they might not trust your message. Do you want to make a pop music video? No one really cares which way you say it.

Exactly! 'How is you ?' is OK here ;) But in a Time magazine article you can't say : " Harvard University sponsors was surprised some years ago when they arranged a ......."

Errr... is this a rhetorical question? It comes down to knowing who, or what, is performing the action. Lucky for you, English verb conjugation is not as extensive as other languages (and even then, those other languages follow a certain pattern of conjugation).

Because if you don't say it right, it is WRONG!