if you want to vote
No, in an election year, primary elections are held in order to pick the party candidates for the general election which is held in November (Although primary often means most important, sometimes, as in this case, primary means first.)
No, Pennsylvania has a "closed primary," meaning that in order to vote for a Presidential primary candidate, one must be registered in that particular party. See chart here: <a href="http://www.primarilypaul.com/ron-paul-in-the-primaries/">Primary Guide</a>
In ORDER FOR A DEMOCRACY TO WORK, PEOPLE HAVE TO DO THEIR DUTIES
No. While exercising your right to vote more often than every 4 years is awesome, you don't have to vote in any one election to qualify to vote in the next one. If anyone tried to prevent you from voting on the basis of your voting history, they are violating your right to vote. You should calmly state that it has no bearing on your right to vote today, and that you intend to do so. If the person continues to obstruct you, call your local officials and insist that you be allowed to vote. Anyone who tries to stop you from having a voice probably doesn't have your best interests at heart.
Open primary
Some states require a petition with a minimum number of signatures in order to get on their presidential election ballot.
Yes, Arizona uses an open primary election system, which means that voters are not required to register with a political party in order to participate in the primary election. Voters may choose to vote in either the Republican or Democratic primary, regardless of their party affiliation.
The 1876 US presidential election saw US Grant to decide not to run for a third term, however, he wanted to make sure the election was fair and balanced. He called upon his friend, General William T. Sherman to guard election returns and to keep order in disputed voting districts.
They call this a closed primary. However, all you have to do is state that youfavor some one party's candidate(s) in order to vote for their slate. You do not have to actually have any proven ties to a party and you can vote however you like in the real election.
Nebraska is a closed primary state. You need to declare your party affiliation in order to vote in primary elections. If you register without a political party affiliation, you will receive only the non-partisan ballots at a primary election. Non-partisans may designate a party preference at primary elections and receive partisan ballots for the Senate and House congressional races. Party designation has no bearing when voting a non-partisan ballot or in the general election.
As of February 2016, Bernie Sanders appears to be leading in most demographics of the Democratic Party. Whatever his current strategy is, it seems to be the one most likely to propel him to victory in the Democratic primary. Whether it will serve him well in a Presidential election depends entirely on which Republican is his adversary.--Update--As of March 2016, it is clear that Bernie Sanders is not effectively courting the Black and Latino communities. In order to be victorious, he would have to embrace the kind of race-baiting that Hillary Clinton engages in, in order to bring these groups on his side. Ultimately the last few primaries that he lost were lost on account of the Black vote.
Vice presidential candidates are generally chosen in order to garner extra votes in the general election. The votes will usually come from a specific area of the nation like the South or the West.