articles

You're missing a full stop (or period if you're in the U.S.). So no, when using letters as an acronym or initialism, you need to either indicate the abbrev. with a termination mark such as a period or comma (which is vastly becoming more and more acceptable). Another acceptable use would be using all capital letters with no termination points, having the acronym all in capitals is now commonplace to provide the information that it is an ancronym and not a word (eg, USA).

I suspect you're asking about the verb and tense, not a missed period. So yes, you can say, "I went to the U.S. yesterday." The sentence is in simple past tense.

Yes it is correct to use the US, the UK, the USSR.

Yes, it is correct.  U.S. is an abbreviation for United States.