For the two questions concerning apples and pears:
you have two choices - 1. Is there an apple on the table?
2. Are there any apples on the table?
You cannot say: Is there any apple on the table,or are there any apple on the table, because apples are countable nouns so you have singular or plural. The same goes for pears:
1. Is there a pear in the box?
2. Are there any pears in the box?
You cannot say: Is there/are there any pear in the box
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