Visitors to our country treat us as we are unintelligent because we are underdeveloped. Especially if we express opinions they do not agree with....

"Visitors to our country treat us as we are unintelligent because we are underdeveloped."

a) Article Usage: The sentence does not have any articles and does not need them; therefore, there are no errors in article usage.

b) As vs. Like: The word "as" is used incorrectly here but substituting "like" wouldn't really work for formal English, although it would constitute an improvement. The best solution would be correcting it to "as if we were unintelligent" since it is a counterfactual conditional, requiring a subjunctive mood. The error in the way "as" is used does make the sentence syntactically unintelligible.

c) Comma: Commas are used after initial subordinate clauses but are not used when the subordinate clause occurs at the end of the sentence. An additional comma is not needed. 

d) Infinitive: The sentence does not contain an infinitive.