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Kindly notice some sort of change in the second word of each pair :-
(1) Tree ---- Trees..... ....(3) Ox ---- Oxen
(2) Box ---- Boxes...... ....(4) Man ---- Men
Here the if you see you would find that , the first word of each pair denotes 'one' thing , where as second word denotes 'more than one' .
A Noun that denotes one person or thing is said to be in the Singular Number ; as ,
Boy , girl , cow , bird , tree , book , etc...