a) I'm in third form.
To determine the ordinal numeral for the grade or form, we need to look at the position of the number in relation to other forms. If there are two forms before yours, then your form is the third.
b) My friend is in second form too.
Since the friend is also in the same form, we need to use the ordinal numeral for the form, which is the second.
c) Today is the tenth of December.
To find the ordinal numeral for a date, we look at the number and identify its position within the month. Since "tenth" refers to the position of the day in the month, it is written as the ordinal numeral.
d) My mom is forty-fifth year.
In this case, we are talking about your mom's age. To express the age as an ordinal numeral, we add "-th" to the number. Therefore, "forty-fifth" means that your mom is 45 years old.
e) Today is the fifth day of the week.
Similar to the previous example, we look at the number and identify its position within the week. Since "fifth" refers to the position of the day in the week, it is written as the ordinal numeral.
f) He is the fifteenth student in our class.
To express the position of a person in a group or class, we use ordinal numerals. In this case, "fifteenth" indicates that he is the 15th student in your class.