Ferdinand de Saussure is considered the father of modern linguistics because of his groundbreaking work in structural linguistics, particularly in his book "Course in General Linguistics." He introduced key concepts such as the signifier and signified, language as a system of differences, and the distinction between synchronic and diachronic linguistics, which had a profound impact on the field of linguistics. His ideas laid the foundation for structuralism and semiotics in the study of language and communication.