Is Krio just poorly spoken British English?
No, Krio is distinct from a pidgin as it is a language in its own right, with fixed grammatical structures and rules. Krio also draws from other European languages, like Portuguese and French, e.g. the Krio word gentri/gentree, which means wealth or to acquire wealth, is derived from the Old French word "gentry," and the Krio word pikin, which means child, indirectly comes from the Portuguese word "pequeno" (meaning "small" and often used to mean children in Portuguese) and several African tribal languages.
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What is the difference between a creole and a dialect?
A dialect is a (typically regional, also social) variant of a language. A creole is something entirely different: Creoles are separate languages, which use the words of some different language (often English or French), but have a grammar that has only little resemblance to that "master" language.