زائدتان means that the alif and nun are not part of the root letters of the word. First recognize that they are additional; then check which of the three taught situations applies.
The visible ending alone cannot prove this. In بُسْتَانٌ, the final ان belongs to the lexical form and is not the additional alif-nun meant by this rule, so the word remains declinable.
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Cause of Ghayr Munsarif: Additional Alif and Nun
زائدتان means that the alif and nun are not part of the root letters of the word. First recognize that they are additional; then check which of the three taught situations applies.
The visible ending alone cannot prove this. In بُسْتَانٌ, the final ان belongs to the lexical form and is not the additional alif-nun meant by this rule, so the word remains declinable.