If I have money, I would buy a CD
Не могу сказать точно но:
A quatation for Madras please send us a shipment
Shall I close the door?
You should see a doctor.
He wasn't able to ride a bike when he was four.
You mustn't park here.
You needn't finish it today.
She must be Brian's sister.
He might come by train.
You ought to work harder.
Children это множественное число.
Детей сосчитать можно, и есть форма единственного числа - child.
Так, что это исчисляемое.