Tell the difference of words I

could versus would:
would is a little more formal.
Would you please give me that pen?
Could you please give me that pen?
center versus centre:
center and centre are identical in meaning,
center is an American spelling.
centre is a British spelling.
for example, theater is an American spelling, but theatre is a British spelling.
to have versus to have got
to have got is informal, to have is a little more formal.
-How much money have you got?
-How much money do you have?

-I have a car.
-I've got a car.

-I have to go.
-I've got to go.

Should we use singular or plural verbs with percentages.

30% of the population (is/are) single.
You have to look at what comes after the word OF.
If the object is singular, then the verb is singular.
If the object is plural, then the verb is pluarl.
-30% of the pie was missing.
-30% of the pies were missing.