Question

Which basic principle that we use today was established by the Magna Carta?

Answer 1

People accused of a crime have the right to be judged by a jury of peers before receiving punishment.

Here is the clause from the Magna Carta which states that principle:

Clause 39: No free man shall be seized or imprisoned, or stripped of his rights or possessions, or outlawed or exiled, or deprived of his standing in any other way, nor will we proceed with force against him, or send others to do so, except by the lawful judgement of his equals or by the law of the land.

That assertion by The Magna Carta of 1215, that a person should not suffer punishment without the lawful judgment of his peers, influenced the Bill of Rights of the US Constitution. The opening lines of the 6th Amendment of the Constitution of the United States affirm that "in all criminal prosecutions, the accused shall enjoy the right to a speedy and public trial, by an impartial jury of the State and district wherein the crime shall have been committed."

Answer 2

Magna Carta is a cornerstone of the individual liberties that we enjoy, and it presents an ongoing challenge to arbitrary rule. 
The continuing importance of Magna Carta as a source of liberty is well established. 
Hope i helped!