Gravitational waves offer a powerful probe of the early universe, providing insights into its dynamics and fundamental physics. This talk will explore the stochastic gravitational wave background (SGWB) and its potential origins, including signals detected in the NANOGrav 15-year dataset. I will discuss how hybrid inflation can generate topological defects, such as cosmic strings, and contribute to the SGWB, highlighting model-building approaches that link theory to observation. Cosmic Strings may have an imprint on the CMB. I will briefly talk about how we can see their footprints in the CMB and constrain the cosmic string tension. Additionally, we will examine the possible formation of primordial black holes. This presentation aims to bridge theoretical insights with observational data, illuminating the connections between inflation, gravitational waves, and cosmic structure formation.