How do we learn about a civilization if we can't read its writings?

It can be difficult to learn much about a culture if we can't access their written records. Whether it be bureaucratic or administrative documents, or historical narratives (like those by Herodotus or Polybius for example), written sources serve as the cornerstone of history as a discipline, precisely because written records are fixed within time: and give one a direct insight into the past, whether it be social, or cultural, or (as in the case of tax records) bureaucratic. When we have access to the writings, we have a direct window into the culture, society and people that produced them. Without this, things become more difficult. That being said, there are other sources of insight out there, some of which can be quite insightful.
First, there are cultures and societies that have rich storytelling traditions, and recite their histories and stories generation by generation across time (indeed, some of the most significant works of human culture have their origins as part of an Oral Tradition: the Homeric Epics, the Torah, the Vedas, etc). In addition, we can also turn towards more purely archaeological resources: through excavation, we can uncover the physical remains of distant civilizations: the ruins of cities, tools and pottery, burial sites, etc. This also can reveal a wealth of information concerning the distant past.