Complex sentence: A matrix\\ Phi (t) is called a principal fundamental matrix solution if all columns are linearly independent solutions and there exists t_0 such that\\ Phi (t_0) is the identity.
Split into simpler sentences: A matrix\\ phi (t) is called a fundamental matrix solution if all columns are linearly independent solutions. It is called a principal fundamental matrix at t_0 if\\ phi (t_0) is the identity.