The Spanish verb 'llenar' (to fill) comes from the Latin word 'plenus' meaning 'full' or 'complete'. First, 'plenus' evolved into the Spanish adjective 'lleno' (full). Then, by adding the common Spanish verb suffix '-ar' to 'lleno', the verb 'llenar' was formed, meaning 'to fill' or literally 'to make full'.

The initial Latin 'pl-' sound commonly evolved into 'll-' in Spanish, which explains the spelling change from 'plenus' to 'lleno'.

The Spanish adjective 'lleno' (full) is directly related to 'llenar' since the verb was derived from it. When you learn 'llenar', it's helpful to remember its connection to 'lleno' - when you 'llenar' something, you make it 'lleno' (full).

Several English words share the same Latin root 'plenus', including 'plenty', 'plenary', 'plenitude', and 'replenish'. All these words carry the basic meaning of 'fullness' or 'completeness'. For example, when you 'replenish' supplies, you fill them back up, and when you have 'plenty' of something, you have a full amount.