Where vocabulary is the main concern, you can write casual (spoken) English if you like, or you can write wrtten (formal) English; it depends on the recipient, the situation, and the purpose. Where they differ more than in vocabulary is in structure-- spoken English is linear, and it can be rambling, often very unstructured, and interrupted by pauses, repetitions, and changes of direction. This never occurs in written English except when the spoken language is being transcribed.
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