Suppose a phrase is easy to understand. That fact alone does not establish it as muhkam. Zahir denotes readily apparent meaning; nass adds connection to the passage’s particular purpose; mufassar eliminates the relevant interpretive uncertainty; muhkam also concerns an absence of susceptibility to abrogation in this scheme.
Instead of memorizing labels alone, ask which feature justifies each classification: immediate meaning, contextual purpose, remaining interpretive possibilities, or enduring status.