should

The word 'should' originates from the Old English 'sceolde', the past tense of 'sculan', meaning "to owe, be obliged to, have to". This Old English verb is derived from the Proto-Germanic skulaną, which carried a sense of obligation or necessity. The semantic core of debt and obligation inherent in the Germanic root evolved into the modern modal meanings of duty, advisability, and expectation. Over centuries, 'sceolde' underwent regular sound changes, such as the loss of the 'c' sound and the hardening of 'ld', to become the modern 'should', while its function specialized within the English modal verb system.

The word 'should' is a single, unsegmentable grammatical unit in modern English. It is the past tense form of the modal verb 'shall'. Its functional role is purely grammatical, expressing modality.