Complex sentence: Martínez, then ranked "piloto primero" and "alférez de navío" (Ensign or Sub-Lieutenant), lead an expedition that arrived at Nootka Sound on May 5,1789.
Split into simpler sentences: Martínez, then ranked "piloto primero" and "alférez de navío" (Ensign or Sub-Lieutenant), lead an expedition that arrived at Nootka Sound in early 1789. The Martínez expedition arrived at Nootka Sound on May 5,1789.