1. The problem of language change: Sweet recognized that languages are constantly evolving, with new words appearing, old words changing meaning, and grammatical structures shifting. He saw this as a challenge for grammarians, who needed to account for the dynamism of language while still providing a clear and consistent description.
2. The problem of individual variation: Sweet also acknowledged that even within a single language, people speak differently. He observed variations in pronunciation, vocabulary, and grammatical structures, all dependent on factors like region, social class, and personal style. He argued that grammarians needed to recognize this variation and provide a comprehensive account of the different ways a language can be used.
Sweet's approach to these problems was based on his belief that grammar should be descriptive rather than prescriptive. He argued that grammarians should focus on describing how language is actually used, rather than trying to impose arbitrary rules on speakers. He believed that understanding the dynamics of language change and the diversity of individual usage was key to understanding the nature of grammar.
In essence, Sweet saw the challenge of grammar as understanding and describing the complex and dynamic nature of language, recognizing both its evolution and its individual variations.