The Hellenic branch of Indo-European languages consists almost solely of Greek. The only other living languages in that branch are subsets of Greek.
Greek has influenced the English language quite a bit. It has done this because it was, for a long time, one of the languages (along with Latin) that all educated people were expected to learn. Therefore, many of our academic terms come from Greek. For example, subject names such as Geology and Biology come from Greek. So do Economics and Philosophy. Greek has also contributed terms to our language of technology -- such as telegraph and telegram.