The Spanish word 'hierba' meaning 'herb' or 'grass' comes directly from the Latin word 'herba', which had the same meaning. During the evolution from Latin to Spanish, the 'e' in 'herba' underwent a common sound change called diphthongization, where it became 'ie', resulting in 'hierba'. This type of vowel change was very common in the development of Spanish from Latin.

A common variant spelling of 'hierba' in Spanish is 'yerba', which you might recognize from 'yerba mate', the popular South American tea. Both spellings refer to the same word and share the same Latin origin.

English speakers can easily remember 'hierba' by connecting it to the English word 'herb', which comes from the same Latin root 'herba'. While English dropped the 'h' sound in 'herb' (in most dialects), the meaning remains very similar to the Spanish word, referring to plants used for food, medicine, or flavoring.