Dear Mr. Verb,
I would like to be clarified about an issue regarding the verb to have. Is it possible to use 'have' when speaking about a country? Ex. Russia have accomplished many important tasks... Australia have beaten New Zealand ... . The apparent contradiction is that a country is both a single entity and a community. But I would like to confirm that it is so, and that it is in fact possible to use 'have' in these situations.
Ahhhh, finally, a question I can answer even in a hurry.
This is a major difference in national varieties of English, and it's not just to have, but applies to all verbs. The issue is exactly the one you've noted: whether we treat a particular noun as a single entity or a collection of individuals, so have singular or plural verb agreement. In the US, collective nouns tend to be singular; British English speakers treat many such cases as plurals: the team (like for 'Australia', above), the band, the gang, the committee is / are.
On any question of US/UK difference, Separated by a Common Language is the place to turn and Lynneguist has posted about this, here. She notes some variation in US usage, like with jury, but the difference seems pretty robust most of the time.